A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public
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Title: A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public
Author: Frank Bertram Wade
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By Frank B. Wade
Diamonds
A Text-Book of Precious Stones
A TEXT-BOOK OF PRECIOUS STONES FOR JEWELERS AND THE GEM-LOVING PUBLIC
by
FRANK B. WADE, B.S.
Head of the Department of Chemistry, Shortridge High
School, Indianapolis, Ind.
Author of "Diamonds: A Study of the Factors That
Govern Their Value"
Illustrated
G. P. Putnam's Sons
New York and London
The Knickerbocker Press
Copyright, 1918
by
Frank B. Wade
First printing, January, 1918
Second " March, 1924
[Device]
Made in the United States of America
PREFACE
In this little text-book the author has tried to combine the trade
information which he has gained in his avocation, the study of precious
stones, with the scientific knowledge bearing thereon, which his
vocation, the teaching of chemistry, has compelled him to master.
In planning and in writing the book, every effort has been made to teach
the fundamental principles and methods in use for identifying precious
stones, in as natural an order as possible. This has been done in the
belief that the necessary information will thus be much more readily
acquired by the busy gem merchant or jeweler than would have been the
case had the material been arranged in the usual systematic order. The
latter is of advantage for quick reference after the fundamentals of the
subject have been mastered. It is hoped, however, that the method of
presentation used in this book will make easy the acquisition of a
knowledge of gemology and that many who have been deterred from studying
the subject by a feeling that the difficulties due to their lack of
scientific training were insurmountable, will find that they can learn
all the science that is really necessary, as they proceed. To that end
the discussions have been given in as untechnical language as possible
and homely illustrations have in many cases been provided.
Nearly every portion of the subject that a gem merchant needs to know
has been considered and there is provided for the interested public much
material which will enable them to be more intelligent purchasers of
gem-set jewelry, as well as more appreciative lovers of Nature's
wonderful mineral masterpieces.
F. B. W.
INDIANAPOLIS,
_December 26, 1916_
INTRODUCTION
Because of the rapid increase in knowledge about precious stones on the
part of the buying public, it has become necessary for the gem merchant
and his clerks and salesmen to know at least as much about the subject
of gemology as their better informed customers are likely to know.
In many recent articles in trade papers, attention has been called to
this need, and to the provision which Columbia University has made for a
course in the study of gems. The action of the National Association of
Goldsmiths of Great Britain in providing annual examinations in
gemology, and in granting certificates and diplomas to those who
successfully pass the examinations, has also been reported, and it has
been suggested that some such course should be pursued by jewelers'
associations in this country. The greatest difficulty in the way of such
formal study among our jewelers and gem merchants is the lack of time
for attendance on formal courses, which must necessarily be given at
definite times and in definite places.
As a diamond salesman was heard to say recently: "The boss said he
wanted me to take in that course at Columbia, but he didn't tell me how
I was going to do it. Here I am a thousand miles from Columbia, and it
was only six weeks ago that he was telling me I ought to take that
course. I can't stay around New York all the time." Similarly those
whose work keeps them in New York might object that their hours of
employment prevented attendance on day courses, and that distance from
the university and fatigue prevent attendance on night courses. The
great mass of gem dealers in other cities must also be considered.
It will therefore be the endeavor of this book to provide guidance for
those who really want to make themselves more efficient in the gem
business, but who have felt that they needed something in the way of
suggestion regarding what to attempt, and how to go about it.
Study of the sort that will be suggested can be pursued in spare
moments, on street cars or elevated trains, in waiting rooms, or in
one's room at night. It will astonish many to find how much can be
accomplished by consistently utilizing spare moments. Booker T.
Washington is said to have written in such spare time practically all
that he has published.
For the practical study of the gems themselves, which is an absolutely
essential part of the work, those actually engaged in the trade have
better opportunities than any school could give and, except during rush
seasons, there is plenty of time during business hours for such study.
No intelligent employer will begrudge such use of time for which he is
paying, if the thing be done in reason and with a serious view to
improvement. The frequent application of what is acquired, as
opportunity offers, in connection with ordinary salesmanship, will help
fix the subject and at the same time increase sales.
Many gem dealers have been deterred from beginning a study of gems
because of the seeming difficulties in connection with the scientific
determination of the different varieties of stones. Now science is
nothing but boiled-down common sense, and a bold front will soon
convince one that most of the difficulties are more apparent than real.
Such minor difficulties as exist will be approached in such a manner
that a little effort will overcome them. For those who are willing to do
more work, this book will suggest definite portions of particular books,
which are easily available, for reference reading and study--but the
lessons themselves will attempt to teach the essential things in as
simple a manner as is possible.
Perhaps the first essential for the gem merchant is to be able surely to
distinguish the various stones from one another and from synthetic and
imitation stones.
That such ability is much needed will be clear to anyone who in casting
a backward glance over his experience recalls the many serious mistakes
that have come to his knowledge. Many more have doubtless occurred
without detection. Several times recently the author has come across
cases where large dealers have been mistaken in their determination of
colored stones, particularly emeralds. Only the other day a ring was
brought to me that had been bought for a genuine emerald ring after the
buyer had taken it to one of the dealers in his city and had paid for an
examination of it, which had resulted in its being declared genuine. On
examining the stone with a lens of only moderate power, several round
air bubbles were noted in it, and on barely touching it with a file it
was easily scratched. The material was green glass. Now, what was said
about the dealer who sold it and the one who appraised it may be
imagined. The long chain of adverse influence which will be put in
action against those dealers, even though the one who sold the stone
makes good the loss, is something that can be ill afforded by any
dealer, and all this might have been avoided by even a rudimentary
knowledge of the means of distinguishing precious stones. The dealer was
doubtless honest, but, through carelessness or ignorance, was himself
deceived.
Our first few lessons will therefore be concerned chiefly with learning
the best means of telling the different stones from one another.
CONTENTS
PAGE
PREFACE iii
LESSON
I.--HOW STONES ARE DISTINGUISHED FROM ONE ANOTHER 1
II.--REFRACTION 4
III.--DOUBLE REFRACTION 8
IV.--ABSORPTION AND DICHROISM 15
V.--SPECIFIC GRAVITY 23
VI.--SPECIFIC GRAVITY DETERMINATIONS 31
VII.--LUSTER AND OTHER REFLECTION EFFECTS 38
VIII.--HARDNESS 47
IX.--HARDNESS (_Continued_) 55
X.--DISPERSION 60
XI.--COLOR 66
XII.--COLOR (_Continued_) 75
XIII.--COLOR (_Continued_) 87
XIV.--COLOR (_Concluded_) 93
XV.--HOW TO TELL SCIENTIFIC STONES FROM NATURAL GEMS 99
XVI.--HOW TO TEST AN "UNKNOWN" GEM 109
XVII.--SUITABILITY OF STONES FOR VARIOUS TYPES OF
JEWELS, AS DETERMINED BY HARDNESS,
BRITTLENESS, AND CLEAVABILITY 119
XVIII.--MINERAL SPECIES TO WHICH THE VARIOUS GEMS
BELONG AND THE CHEMICAL COMPOSITION THEREOF 133
XIX.--THE NAMING OF PRECIOUS STONES 149
XX.--THE NAMING OF PRECIOUS STONES (_Concluded_) 164
XXI.--WHERE PRECIOUS STONES ARE FOUND 179
XXII.--HOW ROUGH PRECIOUS STONES ARE CUT 201
XXIII.--HOW ROUGH PRECIOUS STONES ARE CUT AND WHAT
CONSTITUTES GOOD "MAKE" (_Concluded_) 213
XXIV.--FORMS GIVEN TO PRECIOUS STONES 227
XXV.--IMITATIONS OF PRECIOUS STONES 237
XXVI.--ALTERATION OF THE COLOR OF PRECIOUS STONES 250
XXVII.--PEARLS 258
XXVIII.--CULTURED PEARLS AND IMITATIONS OF PEARLS 277
XXIX.--THE USE OF BALANCES AND THE UNIT OF WEIGHT IN
USE FOR PRECIOUS STONES 283
XXX.--TARIFF LAWS ON PRECIOUS AND IMITATION STONES 294
BIBLIOGRAPHY 301
INDEX 313
A Text-Book of Precious Stones
LESSON I
HOW STONES ARE DISTINGUISHED FROM ONE ANOTHER
PRECIOUS STONES DISTINGUISHED BY THEIR _PROPERTIES_. One precious stone
is best distinguished from another just as substances of other types are
distinguished, that is to say, by their _properties_. For example, salt
and sugar are both _white_, both are _soluble in water_, and both are
_odorless_. So far the italicized properties would not serve to
distinguish the two substances. But sugar is _sweet_ while salt is
_salty_ in taste. Here we have a distinguishing property. Now, just as
salt and sugar have properties, so have all _precious stones_, and
while, as was the case with salt and sugar, many precious stones have
properties in common, yet each has also some properties which are
distinctive, and which can be relied upon as differentiating the
particular stone from other stones. In selecting properties for use in
distinguishing precious stones, such properties as can be determined by
quantity, and set down in numbers, are probably more trustworthy than
those that can be observed by mere inspection. Those also which have to
do with the behavior of light in passing through the stone are extremely
valuable.
IMPORTANCE OF NUMERICAL PROPERTIES. It is because gem dealers so often
rely upon the more obvious sort of property, such as color, that they so
frequently make mistakes. There may be several different types of stones
of a given color, but each will be found to have its own numerical
properties such as density, hardness, refractive power, dispersive
power, etc., and it is only by an accurate determination of two or three
of these that one can be sure what stone he has in hand. It must next be
our task to find exactly what is meant by each of these numerical
properties, and how one may determine each with ease and exactness.
LESSON II
REFRACTION
EXPLANATION OF REFRACTION. Perhaps the surest single method of
distinguishing precious stones is to find out the _refractive index_ of
the material. To one not acquainted with the science of physics this
calls for some explanation. The term _refraction_ is used to describe
the bending which light undergoes when it passes (at any angle but a
right angle) from one transparent medium to another. For example, when
light passes from air into water, its path is bent at the surface of the
water and it takes a new direction within the water. (See Fig. 1.)
[Illustration: FIG. 1.]
AB represents the path of light in the air and BC its path in the water.
While every gem stone refracts light which enters it from the air, _each
stone has its own definite ability to do this, and each differs from
every other in the amount of bending which it can bring about under
given conditions_. The accurate determination of the amount of bending
in a given case requires very finely constructed optical instruments and
also a knowledge of how to apply a certain amount of mathematics.
However, all this part of the work has already been done by competent
scientists, and tables have been prepared by them, in which the values
for each material are put down.
THE HERBERT-SMITH REFRACTOMETER. There is on the market an instrument
called the Herbert-Smith refractometer, by means of which anyone with a
little practice can read at once on the scale within the instrument the
_refractive index_, as it is called, of any precious stone that is not
too highly refractive. (Its upper limit is 1.80. This would exclude very
few stones of importance, _i. e._, zircon, diamond, sphene, and
demantoid garnet.)
Those readers who wish to make a more intensive study of the
construction and use of the refractometer will find a very full and
complete account of the subject in _Gem-Stones and their Distinctive
Characters_, by G. F. Herbert-Smith, New York; James Pott & Co., 1912.
Chapter IV., pp. 21-36. The Herbert-Smith refractometer is there
described fully, its principle is explained and directions for using it
are given. The price of the refractometer is necessarily so high (duty
included) that its purchase might not be justified in the case of the
smaller retailer. Every large dealer in colored stones, whether
importer, wholesaler, or retailer, should have one, as by its use very
rapid and very accurate determinations of stones may be made, and its
use is not confined to unmounted stones, for any stone whose table facet
can be applied to the surface of the lens in the instrument can be
determined.
LESSON III
DOUBLE REFRACTION
EXPLANATION OF DOUBLE REFRACTION. In Lesson II. we learned what is meant
by _refraction_ of light. While glass and a small number of precious
stones (diamond, garnet, and spinel) bend light as was illustrated in
Fig. 1, practically all the other stones cause a beam of light on
entering them to separate, and the path of the light in the stone
becomes double, as shown in Fig. 2.
This behavior is called _double refraction_. It may be used to
distinguish those stones which are doubly refracting from those which
are not. For example, in the case of a stone which is doubly refracting
to a strong degree, such as a peridot (the lighter yellowish-green
chrysolite is the same material and behaves similarly toward light),
the separation of the light is so marked that the edges of the rear
facets, as seen through the table, appear _double_ when viewed through a
lens. A zircon will also similarly separate light and its rear facets
also appear double-lined as seen with a lens from the table of the
stone. The rarer stones, sphene and epidote, likewise exhibit this
property markedly. Some colorless zircons, when well cut, so closely
resemble diamonds that even an expert might be deceived, if caught off
his guard, but this simple test of looking for the doubled lines at the
back of the stone would alone serve to distinguish the two stones.
[Illustration: FIG. 2.]
A SIMPLE BUT VERY VALUABLE TEST FOR THE KIND OF REFRACTION OF A CUT
STONE. In the case of most of the other doubly refracting stones the
degree of separation is much less than in peridot and zircon, and it
takes a well-trained and careful eye to detect the doubling of the
lines. Here a very simple device will serve to assist the eye in
determining whether a cut stone is singly or doubly refracting. Expose
the stone to direct sunlight and hold an opaque white card a few inches
from the stone, in the direction of the sun, so as to get the bright
reflections _from within the stone_ reflected onto the card.
If the material is singly refractive (as in the case of diamond, garnet,
spinel, and glass), _single images_ of each of the reflecting facets
will appear on the card, but if doubly refracting--even if slightly
so--_double images_ will appear. When the stone is slightly moved,
these pairs of reflections will travel _together as pairs_ and not tend
to separate. The space between the two members of each pair of
reflections serves to give a rough idea of the degree of the double
refraction of the material if compared with the space between members in
the case of some other kind of stone held at the same distance from the
card. Thus zircon separates the reflections widely. Aquamarine, which is
feebly doubly refracting, separates them but slightly.
It will be seen at once that we have here a very easily applied test and
one that requires no costly apparatus. It is, furthermore, a sure test,
after a little practice. For example, if one has something that looks
like a fine emerald, but that may be glass, all one need to do is to
expose it in the sun, as above indicated. If real emerald, double images
will be had (very close together, because emerald is but feebly doubly
refracting). If glass, the images on the card will be single.
Similarly, ruby can at once be distinguished from even the finest garnet
or ruby spinel, as the last two are singly refracting. So, too, are
glass imitations of ruby and ruby doublets (which consist of glass and
garnet). This test cannot injure the stone, it may be applied to mounted
stones, and it is reliable. For stones of very deep color this test may
fail for lack of sufficiently brilliant reflections. In such a case hold
the card _beyond_ the stone and let the sunlight shine _through_ the
stone onto the card, observing whether the spots of light are single or
double.
The table below gives the necessary information as to which stones show
double and which single refraction.
TABLE GIVING CHARACTER OF REFRACTION IN THE PRINCIPAL GEMS
_Refraction Single:_
Diamond
Garnet (all types)
Spinel
Opal
Glass
_Difference between
highest and lowest
refractive indices_
_Refraction Double:_
Sphene .084
Zircon .053
Benitoite .047
Peridot or chrysolite .038
Epidote .031
Tourmaline .020
Kunzite .015
Ruby and sapphire .009
Topaz (precious) .009
Amethyst and quartz topaz .009
Emerald and aquamarine .007
Chrysoberyl .007
The student should now put into practice the methods suggested in this
lesson. Look first for the visible doubling of the lines of the back
facets in peridot (or chrysolite); then in zircon; then in some of the
less strongly doubly refracting stones; then try the sunlight-card
method with genuine stones and with doublets and imitations until you
can tell every time whether you are dealing with singly or doubly
refracting material. When a stone of unknown identity comes along, try
the method on it and thus assign it as a first step to one or the other
class. Other tests will then be necessary to definitely place it.
DIFFERENCES IN REFRACTION DUE TO CRYSTAL FORM. The difference in
behavior toward light of the singly and doubly refracting minerals
depends upon the crystal structure of the mineral. All gems whose
crystals belong in the cubic system are singly refracting in all
directions: In the case of some other systems of crystals the material
may be singly refracting in one or in two directions, but doubly
refracting in other directions. No attention need be paid to these
complications, however, when using the sunlight-card method with a cut
stone, for in such a case the light in its course within the stone will
have crossed the material in two or more directions, and the separation
and consequent doubling of image will be sure to result. For those who
wish to study double refraction more in detail, Chapter VI., pages
40-52, of G. F. Herbert-Smith's _Gem-Stones_ will serve admirably as a
text. As an alternative any text-book on physics will answer.
LESSON IV
ABSORPTION AND DICHROISM
CAUSE OF COLOR IN MINERALS. In Lesson III. we saw that many gem
materials cause light that enters them to divide and take two paths
within the material. Now all transparent materials _absorb_ light more
or less; that is, they stop part of it, perhaps converting it into heat,
and less light emerges than entered the stone. If light of all the
rainbow colors (red, orange, yellow, green, blue, violet) is equally
absorbed, so that there is the same relative amount of each in the light
that comes out as in the light that went into a stone, we say that the
stone is a _white_ stone; that is, it is not a _colored_ stone. If,
however, only blue light succeeds in getting through, the rest of the
white light that entered being absorbed within, we say that we have a
blue stone.
Similarly, the _color_ of any transparent material depends upon its
relative degree of absorption of each of the colors in white light. That
color which emerges most successfully gives its name to the color of the
stone. Thus a ruby is red because red light succeeds in passing through
the material much better than light of any other color.
UNEQUAL ABSORPTION CAUSES DICHROISM. All that has been said so far
applies equally well to both singly and doubly refracting materials, but
in the latter sort it is frequently the case, in those directions in
which light always divides, that the absorption is not equal in the two
beams of light (one is called the ordinary ray and the other the
extraordinary ray).
For example, in the case of a crystal of ruby, if white light starts to
_cross_ the crystal, it not only divides into an ordinary ray and an
extraordinary ray, but the absorption is different in the two cases,
and the two rays emerge of different shades of red. With most rubies one
ray emerges purplish red, the other yellowish red.
It will at once be seen that if the human eye could distinguish between
the two rays, we would have here a splendid method of determining many
precious stones. Unfortunately, the eye does not analyze light, but
rather blends the effect so that the unaided eye gives but a poor means
of telling whether or not a stone exhibits twin colors, or _dichroism_,
as it is called. (The term signifies two colors.) A well-trained eye
can, however, by viewing a stone in several different positions, note
the difference in shade of color caused by the differential absorption.
THE DICHROSCOPE. Now, thanks to the scientific workers, there has been
devised a relatively simple and comparatively inexpensive instrument
called the _dichroscope_, which enables one to tell almost at a glance
whether a stone is or is not dichroic. The construction is indicated in
the accompanying drawing and description.
[Illustration: THE DICHROSCOPE.]
[Illustration: FIG. 3.
A, simple lens; B, piece of Iceland spar with glass prisms on ends to
square them up; C, square hole.]
[Illustration: FIG. 4.]
If the observer looks through the lens (A) toward a bright light, as,
for example, the sky, he apparently sees two square holes, Fig. 4.
What has happened is that the light passing through the square hole (C
of Fig. 3) has divided in passing through the strongly doubly refracting
Iceland spar (B of Fig. 3) and two images of the square hole are thus
produced.
If now a stone that exhibits dichroism is held in front of the square
hole and viewed toward the light, two images of the stone are seen, one
due to its ordinary ray (which, as was said above, will have one color),
and the other due to its extraordinary ray (which will have a different
color or shade of color), thus the color of the two squares will be
different.
With a singly refracting mineral, or with glass, or with a doubly
refracting mineral when viewed in certain directions of the crystal
(which do not yield double refraction) the colors will be alike in the
two squares. Thus to determine whether a red stone is or is not a ruby
(it might be a garnet or glass or a doublet, all of which are singly
refracting and hence can show no dichroism), hold the stone before the
hole in the dichroscope and note whether or not it produces twin colors.
If there seems to be no difference of shade turn the stone about, as it
may have accidentally been placed so that it was viewed along its
direction of single refraction. If there is still no dichroism it is not
a ruby. (_Note._--Scientific rubies exhibit dichroism as well as natural
ones, so this test will not distinguish them.)
A dichroscope may be had for from seven to ten dollars, according to the
make, and everyone who deals in colored stones should own and use one.
Not all stones that are doubly refracting exhibit dichroism. White
stones of course cannot exhibit it even though doubly refracting, and
some colored stones, though strongly doubly refracting, do not exhibit
any noticeable dichroism. The zircon, for example, is strongly doubly
refracting, but shows hardly any dichroism.
The test is most useful for emerald, ruby, sapphire, tourmaline,
kunzite and alexandrite, all of which show marked dichroism.
It is of little use to give here the twin colors in each case as the
shades differ with different specimens, according to their depth and
type of color. The deeper tinted stones of any species show the effect
more markedly than the lighter ones.
The method is rapid and easy--it can be applied to mounted stones as
well as to loose ones, and it cannot injure a stone. The student should,
if possible, obtain the use of a dichroscope and practice with it on all
sorts of stones. He should especially become expert in distinguishing
between rubies, sapphires, and emeralds, and their imitations. The only
imitation (scientific rubies and sapphires are not here classed as
imitations), which is at all likely to deceive one who knows how to use
the dichroscope is the emerald triplet, made with real (but pale) beryl
above and below, with a thin strip of green glass between. As beryl is
doubly refracting to a small degree, and dichroic, one might perhaps be
deceived by such an imitation if not careful. However, the amount of
dichroism would be less in such a case than in a true emerald of as deep
a color.
Those who wish to study further the subject of dichroism should see
_Gem-Stones_, by G. F. Herbert-Smith, Chapter VII., pp. 53-59, or see _A
Handbook of Precious Stones_, by M. D. Rothschild, Putnam's, pp. 14-16.
LESSON V
SPECIFIC GRAVITY
The properties so far considered as serving to distinguish precious
stones have all depended upon the behavior of the material toward light.
These properties were considered first because they afford, to those
acquainted with their use, very rapid and sure means of classifying
precious stones.
DENSITY OF MINERALS. We will next consider an equally certain test,
which, however, requires rather more time, apparatus, and skill to
apply.
Each kind of precious stone has its own _density_. That is, if pieces of
_different_ stones were taken all of the same size, the _weights_ would
differ, but like-sized pieces of one and the same material always have
the same weight. It is the custom among scientists to compare the
densities of substances with the density of water. The number which
expresses the relation between the density of any substance and the
density of water is called the _specific gravity_ number of the
substance. For example, if, size for size, a material, say diamond, is
3.51 times as heavy as water, its _specific gravity_ is 3.51. It will be
seen that since each substance always has, when pure, the same _specific
gravity_, we have here a means of distinguishing precious stones. It is
very seldom, if ever, the case that we find any two precious stones of
the same specific gravity. A few stones have nearly the same specific
gravities, and in such cases it is well to apply other tests also. _In
fact one should always make sure of a stone by seeing that two or three
different tests point to the same species._
We must next find out how to determine the specific gravity of a
precious stone. If the shape of a stone were such that the volume could
be readily calculated, then one could easily compare the weight with the
volume or with the weight of the same volume of water, and thus get the
specific gravity (for a specific gravity number really tells how much
heavier a piece of material is than the same volume of water).
Unfortunately the form of most precious stones is such that it would be
very difficult to calculate the volume from the measurements, and the
latter would be hard to make accurately with small stones. To avoid
these difficulties the following ingenious method has been devised:
If a stone is dropped into water it pushes aside, or _displaces_, a body
of water exactly equal in volume to itself. If the water thus displaced
were caught and weighed, and the weight of the stone then divided by the
weight of the water displaced, we would have the specific gravity number
of the stone.
This is precisely what is done in getting the specific gravity of small
stones. To make sure of getting an accurate result for the weight of
water displaced the following apparatus is used.
[Illustration: FIG. 5.
A, Flask-like Bottle; B, Indicates Ground Glass Stopper; C, Shows Hole
Drilled through Stopper.]
THE SPECIFIC GRAVITY BOTTLE. A small flask-like bottle (see Fig. 5) is
obtained. This has a tightly fitting _ground_ glass stopper (B). The
stopper has a small hole (C) drilled through it lengthwise. If the
bottle is filled with water, and the stopper dropped in and tightened,
water will squirt out through the small hole in the stopper. On wiping
off stopper and bottle we have the bottle _exactly full_ of water. If
now the stopper is removed, the stone to be tested (which must of course
be smaller than the neck of the bottle) dropped in, and the stopper
replaced, exactly as much water will squirt out as is equal in volume to
the stone that was dropped in.
If we had weighed the full bottle with the stone _on the pan beside it_,
and then weighed the bottle with the stone _inside it_ we could now, by
subtracting the last weight from the first, find out how much the water,
that was displaced, weighed. This is precisely the thing to do. The
weight of the stone being known we now have merely to divide the weight
of the stone by the weight of the displaced water, and we have the
specific gravity number. Reference to a table of specific gravities of
precious stones will enable us to name our stone. Such a table follows
this lesson.
A SAMPLE CALCULATION. The actual performance of the operation, if one is
skilled in weighing, takes less time than it would to read this
description. At first one will be slow, and perhaps one should read and
re-read this lesson, making sure that all the ideas are clear before
trying to put them in practice.
A sample calculation may help make the matter clearer, so one is
appended:
Weight of bottle + stone (outside) = 53.51 carats
Weight of bottle + stone (inside) = 52.51 carats
------------
Weight of water displaced = 1.00 carat
------------
Weight of stone = 3.51 carats
Weight of stone 3.51
Specific gravity = --------------- = ---- = 3.51 Sp. g.
Weight of water 1.00
In this case the specific gravity being 3.51, the stone is probably
diamond (see table), but might be precious topaz, which has nearly the
same specific gravity.
It is assumed that the jeweler will weigh in carats, and that his
balance is sensitive to .01 carat. With such a balance, and a specific
gravity bottle (which any scientific supply house will furnish for less
than $1) results sufficiently accurate for the determination of
precious stones may be had if one is careful to exclude air bubbles from
the bottle, and to wipe the outside of the bottle perfectly dry before
each weighing. The bottle should never be held in the warm hands, or it
will act like a thermometer and expand the water up the narrow tube in
the stopper, thus leading to error. A handkerchief may be used to grasp
the bottle.
TABLE OF SPECIFIC GRAVITIES OF THE PRINCIPAL GEM MATERIALS
Beryl (Emerald) 2.74
Chrysoberyl (Alexandrite) 3.73
Corundum (Ruby, sapphire, "Oriental topaz") 4.03
Diamond 3.52
Garnet (Pyrope) 3.78
" (Hessonite) 3.61
" (Demantoid, known in the trade as "Olivine") 3.84
" (Almandite) 4.05
Opal 2.15
Peridot 3.40
Quartz (Amethyst, common topaz) 2.66
Spinel (Rubicelle, Balas ruby) 3.60
Spodumene (Kunzite) 3.18
Topaz (precious) 3.53
Tourmaline 3.10
Turquoise 2.82
Zircon, lighter variety 4.20
" heavier variety 4.69
For a more complete and scientific discussion of specific gravity
determination see _Gem-Stones_, by G. F. Herbert-Smith, Chapter VIII.,
pp. 63-77; or see, _A Handbook of Precious Stones_, by M. D. Rothschild,
pp. 21-27, for an excellent account with illustrations; or see any
physics text-book.
LESSON VI
SPECIFIC GRAVITY DETERMINATIONS
WEIGHING A GEM IN WATER. In the previous lesson it was seen that the
identity of a precious stone may be found by determining its specific
gravity, which is a number that tells how much heavier the material is
than a like volume of water. It was not explained, however, how one
would proceed to get the specific gravity of a stone too large to go in
the neck of a specific gravity bottle. In the latter case we resort to
another method of finding how much a like volume of water weighs. If the
stone, instead of being dropped into a perfectly full bottle of water
(which then overflows), be dropped into a partly filled glass or small
beaker of water, just as much water will be displaced as though the
vessel were full, and it will be displaced _upward_ as before, for lack
of any other place to go. Consequently its weight will tend to buoy up
or float the stone by trying to get back under it, and the stone when in
water will weigh less than when in air. Anyone who has ever pulled up a
small anchor when out fishing from a boat will recognize at once that
this is the case, and that as the anchor emerges from the water it seems
to suddenly grow heavier. Not only does the stone weigh less when in the
water, but it weighs exactly as much less as the weight of the water
that was displaced by the stone (which has a volume equal to the volume
of the stone). If we weigh a stone first in the air, as usual, and then
in water (where it weighs less), and then subtract the weight in water
from the weight in air we will have the _loss of weight in water_, and
this equals the _weight of an equal volume of water_, which is precisely
what we got by our bottle method.
We now need only divide the weight in air by the loss of weight in
water, and we shall have the specific gravity of the stone.
[Illustration: FIG. 6.]
To actually weigh the stone in water we must use a fine wire to support
the stone. We must first find how much this wire itself weighs (when
attached by a small loop to the hook that supports the balance pan and
trailing partly in the water, as will be the case when weighing the
stone in water). This weight of the wire must of course be deducted to
get the true weight of the stone in water. The beaker of water is best
supported by a small table that stands over the balance pan. One can
easily be made out of the pieces of a cigar box. (See Fig. 6.)
The wire that is to support the stone should have a spiral at the bottom
in which to lay the gem, and this should be so placed that the latter
will be completely submerged at all times, but not touching bottom or
sides of the beaker.
Example of data, and calculation, when getting specific gravity by the
method of weighing in water:
Weight of stone = 4.02 carats
-----------
Weight of stone (plus wire) in water = 3.32 carats
Weight of wire = .30 carat
-----------
True weight of stone in water = 3.02 carats
-----------
Loss of weight in water = 1.00 carat
Weight of stone 4.02
Specific gravity = --------------- = ---- = 4.02
Loss in water 1.00
Here the specific gravity, 4.02 would indicate some corundum gem (ruby
or sapphire), and the other characters would indicate at once which it
was.
The student who means to master the use of the two methods given in
Lessons V. and VI. should proceed to practice them with stones of known
specific gravities until he can at least get the correct result to the
first decimal place. It is not to be expected that accurate results can
be had in the second decimal place, with the balances usually available
to jewelers. When the learner can determine specific gravities with some
certainty he should then try unknown gems.
The specific gravity method is of especial value in distinguishing
between the various colorless stones, as, for example, quartz crystal,
true white topaz, white sapphire, white or colorless beryl, etc. These
are all doubly refractive, have no color, and hence no dichroism, and
unless one has a refractometer to get the refractive index, they are
difficult to distinguish. The specific gravities are very different,
however, and readily serve to distinguish them. It should be added that
the synthetic stones show the same specific gravities as their natural
counterparts, so that this test does not serve to detect them.
Where many gems are to be handled and separated by specific gravity
determinations, perhaps the best way to do so is to have several liquids
of known specific gravity and to see what stones will float and what
ones will sink in the liquids. Methylene iodide is a heavy liquid (sp.
g. 3.32), on which a "quartz-topaz," for example, sp. g. 2.66, would
float, but a true topaz, sp. g. 3.53, would sink in it. By diluting
methylene iodide with benzol (sp. g. 0.88) any specific gravity that is
desired may be had (between the two limits 0.88 and 3.32). Specimens of
known specific gravity are used with such liquids and their behavior (as
to whether they sink or float, or remain suspended in the liquid,)
indicates the specific gravity of the liquid. An unknown stone may then
be used and its behavior noted and compared with that of a known
specimen, whereby one can easily find out whether the unknown is heavier
or lighter than the known sample.
An excellent account of the detail of this method is given in G. F.
Herbert-Smith's _Gem-Stones_, pages 64-71, of Chapter VIII., and various
liquids are there recommended. It is doubtful if the practical gem
dealer would find these methods necessary in most cases. Where large
numbers of many different unknown gems have to be determined it would
pay to prepare, and standardize, and use such solutions.
LESSON VII
LUSTER AND OTHER REFLECTION EFFECTS
By the term _luster_ we refer to the manner and degree in which light is
reflected from the _surface_ of a material. Surfaces of the same
material, but of varying degrees of smoothness would, of course, vary in
the vividness of their luster, but the type of variation that may be
made use of to help distinguish gems, depends upon the character of the
material more than upon the degree of smoothness of its surface. Just as
silk has so typical a luster that we speak of it as silky luster, and
just as pearl has a pearly luster, so certain gems have peculiar and
characteristic luster. The diamond gives us a good example. Most diamond
dealers distinguish between real and imitation diamonds at a glance by
the character of the luster. That is the chief, and perhaps the only
property, that they rely upon for deciding the genuineness of a diamond,
and they are fairly safe in so doing, for, with the exception of certain
artificially decolorized zircons, no gem stone is likely to deceive one
who is familiar with the luster of the diamond. It is not to be denied
that a fine white zircon, when finely cut, may deceive even one who is
familiar with diamonds. The author has fooled many diamond experts with
an especially fine zircon, for the luster of zircon does approach,
though it hardly equals, that of the diamond. Rough zircons are
frequently mistaken for diamonds by diamond prospectors, and even by
pickers in the mines, so that some care should be exercised in any
suspicious case, and one should not then rely solely on the luster.
However, in most cases in the trade there is almost no chance of the
unexpected presence of a zircon and the luster test is usually
sufficient to distinguish the diamond. (Zircons are strongly doubly
refractive, as was said in Lesson III. on Double Refraction, and with a
lens the doubling of the back lines may be seen.)
ADAMANTINE LUSTER. The luster of a diamond is called _adamantine_ (the
adjective uses the Greek name for the stone itself). It is keen and cold
and glittering, having a metallic suggestion. A very large per cent. of
the light that falls upon the surface of a diamond at any low angle is
reflected, hence the keenness of its luster. If a diamond and some other
white stone, say a white sapphire, are held so as to reflect at the same
time images of an incandescent light into the eye of the observer, such
a direct comparison will serve to show that much more light comes to the
eye from the diamond surface than from the sapphire surface. The image
of the light filament, as seen from the diamond, is much keener than as
seen from the sapphire. The same disparity would exist between the
diamond and almost any other stone. Zircon comes nearest to having
adamantine luster of any of the other gems. The green garnet that is
called "olivine" in the trade also approaches diamond in luster, hence
the name "demantoid," or diamond like, sometimes applied to it.
VITREOUS LUSTER. The other stones nearly all have what is called
_vitreous_ luster (literally, glass like), yet owing to difference of
hardness, and consequent minute differences in fineness of surface
finish, the keenness of this vitreous luster varies slightly in
different stones, and a trained eye can obtain clues to the identity of
certain stones by means of a consideration of the luster. Garnets, for
example, being harder than glass, take a keener polish, and a glance at
a doublet (of which the hard top is usually garnet and the base of
glass) will show that the light is better reflected from the garnet part
of the top slope than from the glass part. This use of luster affords
the quickest and surest means of detecting a doublet. One can even tell
a doublet inside a show window, although the observer be outside on the
sidewalk, by moving to a position such that a reflection from the top
slope of the stone is to be had. When a doublet has a complete garnet
top no such direct comparison can be had, but by viewing first the top
luster, and then the back luster, in rapid succession, one can tell
whether or not the stone is a doublet.
OILY LUSTER. Certain stones, notably the peridot (or chrysolite) and the
hessonite (or cinnamon stone), have an oily luster. This is possibly due
to reflection of light that has penetrated the surface slightly and then
been reflected from disturbed layers beneath the surface. At any rate,
the difference in luster may be made use of by those who have trained
their eyes to appreciate it. Much practice will be needed before one can
expect to tell at a glance when he has a peridot (or chrysolite) by the
luster alone, but it will pay to spend some spare time in studying the
luster of the various stones.
A true, or "precious" topaz, for example, may be compared with a yellow
quartz-topaz, and owing to the greater hardness of the true topaz, it
will be noted that it has a slightly keener luster than the other stone,
although both have vitreous luster. Similarly the corundum gems (ruby
and sapphire), being even harder than true topaz, take a splendid
surface finish and have a very keen vitreous luster.
Turquoise has a dull waxy luster, due to its slight hardness. Malachite,
although soft, has, perhaps because of its opacity, a keen and sometimes
almost metallic luster.
One may note the luster rapidly, without apparatus and without damage to
the stone. We thus have a test which, while it is not conclusive except
in a very few cases, will supplement and serve to confirm other tests,
or perhaps, if used at first, will suggest what other tests to apply.
Another optical effect that serves to distinguish some stones depends
upon the reflection of light from within the material due to a certain
lack of homogeneity in the substance.
CAUSE OF COLOR IN THE OPAL. Thus the opal is distinguished by the
prismatic colors that emerge from it owing to the effect of thin layers
of material of slightly different density, and hence of different
refractive index from the rest of the material. These thin films act
much as do soap-bubble films, to interfere with light of certain wave
lengths, but to reflect certain other wave lengths and hence certain
colors.
Again, in some sapphires and rubies are found minute, probably hollow,
tube-like cavities, arranged in three sets in the same positions as the
transverse axes of the hexagonal crystal. The surfaces of these tubes
reflect light so as to produce a six-pointed star effect, especially
when the stone is properly cut to a high, round cabochon form, whose
base is parallel to the successive layers of tubes.
STARSTONES, MOONSTONES, CAT'S-EYES. In the moonstone we have another
sort of effect, this time due to the presence of hosts of small twin
crystal layers that reflect light so as to produce a sort of
moonlight-on-the-water appearance _within_ the stone when the latter is
properly cut, with the layers of twin crystals parallel to its base.
Ceylon-cut moonstones are frequently cut to save weight, and may have to
be recut to properly place the layers so that the effect may be seen
equally over all parts of the stone, as set.
Cat's-eye and tiger's-eye owe their peculiar appearance to the presence,
within them, of many fine, parallel, silky fibers. The quartz cat's-eye
was probably once an asbestos-like mineral, whose soft fibers were
replaced by quartz in solution, and the latter, while giving its
hardness to the new mineral, also took up the fibrous arrangement of the
original material. The true chrysoberyl cat's-eye also has a somewhat
similar fibrous or perhaps tubular structure. Such stones, when cut _en
cabochon_, show a thin sharp line of light running across the center of
the stone (when properly cut with the base parallel to the fibers).
This is due to reflection of light from the surfaces of the parallel
fibers. The line of light runs perpendicularly to the fibers.
In these cases (opals, starstones, moonstones, and cat's-eyes) the
individual stone is usually easily distinguished from other kinds of
stones by its peculiar behavior towards light. However, it must be
remembered that other species than corundum furnish starstones (amethyst
and other varieties of quartz, for example), so that it does not follow
that any starstone is a corundum gem. Also the more valuable chrysoberyl
cat's-eye may be confused with the cheaper quartz cat's-eye unless one
is well acquainted with the respective appearances of the two varieties.
Whenever there is any doubt other tests should be applied.
For further account of luster and other types of reflection effects see
_Gem-Stones_, by G. F. Herbert-Smith, Chapter V., pp. 37-39, or _A
Handbook of Precious Stones_, M. D. Rothschild, pp. 17, 18.
LESSON VIII
HARDNESS
Another property by means of which one may distinguish the various gems
from each other is _hardness_. By hardness is meant the ability to
resist scratching. The term "hardness" should not be taken to include
toughness, yet it is frequently so understood by the public. Most hard
stones are more or less brittle and would shatter if struck a sharp
blow. Other hard stones have a pronounced _cleavage_ and split easily in
certain directions. True hardness, then, implies merely the ability to
resist abrasion (_i. e._, scratching).
Now, not only is hardness very necessary in a precious stone in order
that it may _receive_ and _keep_ a fine polish, but the degree in which
it possesses hardness as compared with other materials of known
hardness may be made use of in identifying it.
No scale of _absolute_ hardness has ever come into general use, but the
mineralogist Mohs many years ago proposed the following _relative_
scale, which has been used very largely:
MOHS'S SCALE OF HARDNESS. Diamond, the hardest of all gems, was rated as
10 by Mohs. This rating was purely arbitrary. Mohs might have called it
100 or 1 with equal reason. It was merely in order to represent the
different degrees of hardness by numbers, that he picked out the number
10 to assign to diamonds. Sapphire (and ruby) Mohs called 9, as being
next to diamond in hardness. True topaz (precious topaz) he called 8.
Quartz (amethyst and quartz "topaz") was given the number 7. Feldspar
(moonstone) was rated 6, the mineral apatite 5, fluorspar 4, calcite 3,
gypsum 2, and talc 1.
It may be said here that any mineral in this series, that is of higher
number than any other, will scratch the other. Thus diamond (10) will
scratch all the others, sapphire (9) will scratch any but diamond, topaz
(8) will scratch any but diamond and sapphire, and so on.
It must not be thought that there is any regularity in the degrees of
hardness as expressed by these numbers. The intervals in hardness are by
no means equal to the differences in number. Thus the interval between
diamond and sapphire, although given but one number of difference, is
probably greater than that between sapphire (9) and talc (1). The
numbers thus merely give us an order of hardness. Many gem minerals are,
of course, missing from this list, and most of the minerals from 5 down
to 1 are not gem minerals at all. Few gem materials are of less hardness
than 7, for any mineral less hard than quartz (7) will inevitably be
worn and dulled in time by the ordinary road dust, which contains much
powdered quartz.
In testing a gem for hardness the problem consists in finding out which
of the above minerals is most nearly equal in hardness to the unknown
stone. Any gem that was approximately equal in hardness to a true topaz
(8) would also be said to be of hardness 8. Thus spinel is of about the
same hardness as topaz and hence is usually rated as 8 in hardness.
Similarly opal, moonstone, and turquoise are of about the same hardness
as feldspar and are all rated 6.
Frequently stones will be found that in hardness are between some two of
Mohs's minerals. In that case we add one half to the number of the
softer mineral; thus, peridot, benitoite, and jade (nephrite) are all
softer than quartz (7) but harder than feldspar (6); hence we say they
are 6-1/2 in hardness. Beryl (aquamarine and emerald), garnet
(almandine), and zircon are rated 7-1/2 in hardness, being softer than
true topaz but harder than quartz. A table of the hardness of most of
the commonly known gem-stones follows this lesson.
Having now an idea of what hardness means and how it is expressed, we
must next inquire how one may make use of it in identifying unknown
gems.
HOW TO APPLY THE HARDNESS TEST. In the first place, it is necessary to
caution the beginner against damaging a fine gem by attempting to test
its hardness in any but the most careful manner. The time-honored file
test is really a hardness test and serves nicely to distinguish genuine
gems, of hardness 7 or above, from glass imitations. A well-hardened
steel file is of not quite hardness 7, and glass of various types while
varying somewhat averages between 5 and 6. Hence, glass imitations are
easily attacked by a file. To make the file test use only a _very fine_
file and apply it with a light but firm pressure lengthwise along the
girdle (edge) of the unset stone. If damage results it will then be
almost unnoticeable. Learn to know the _feel_ of the file as it takes
hold of a substance softer than itself. Also learn the _sound_. If
applied to a hard stone a file will slip on it, as a skate slips on ice.
It will not take hold as upon a softer substance.
If the stone is set, press a sharp corner of a broken-ended file gently
against a _back_ facet, preferably high up toward the girdle, where any
damage will not be visible from the front, and move the file very
slightly along the surface, noting by the _feel_ whether or not it takes
hold and also looking with a lens to see if a scratch has been made. Do
not mistake a line of steel, left on a slightly rough surface, for a
true scratch. Frequently on an unpolished girdle of real gem material
the file will leave a streak of steel. Similarly when using test
minerals in accordance with what follows do not mistake a streak of
powder from the yielding test material, for a true scratch in the
material being tested. The safe way is to wipe the spot thus removing
any powder. A true scratch will, of course, persist.
A doublet, being usually constructed of a garnet top and a glass back,
may resist a file at the girdle if the garnet top covers the stone to
the girdle, as is sometimes the case, especially in the smaller sizes.
In this case the back must be tested.
One should never pass a file rudely across the corners or edges of the
facets on any stone that may be genuine, as such treatment really
amounts to a series of light hammer blows, and the brittleness of most
gem stones would cause them to yield, irrespective of their hardness. It
should be remembered that some genuine stones are softer than a file, so
that it will not do to reject as worthless any material that is attacked
by a file. Lapis lazuli (5), sphene (5), opal (6), moonstone (6),
amazonite (6), turquoise (6), peridot (6-1/2), demantoid garnet (6-1/2)
(the "olivine" of the trade), and jade (nephrite) (6-1/2), are all more
or less attacked by a file.
TABLE OF HARDNESS OF THE PRINCIPAL GEM-STONES
10. Diamond.
9-1/2. (Carborundum.)
9. Sapphire and ruby (also all the color varieties of
sapphire).
8-1/2. Chrysoberyl (alexandrite).
8. True topaz and spinel (rubicelle, balas ruby).
7-1/2. Emerald, aquamarine, beryl, Morganite, zircon (jacinth and
true hyacinth and jargoon), almandine garnet.
7-1/4. Pyrope garnet (Arizona ruby, cape ruby), hessonite garnet
(cinnamon stone), tourmaline (various colors vary from 7
to 7-1/2), kunzite (7+).
7. Amethyst, various quartz gems, quartz "topaz," jade
(jadeite).
6-1/2. Peridot (chrysolite), demantoid garnet ("olivine"), jade
(nephrite).
6. Opal, moonstone, turquoise.
5. Lapis lazuli.
LESSON IX
HARDNESS--_Continued_
MINERALS USED IN TESTING HARDNESS. For testing stones that are harder
than a file the student should provide himself with the following set of
materials:
1. A small crystal of carborundum. (Most hardware stores have specimen
crystals as attractive advertisements of carborundum as an abrasive
material, or the Carborundum Co., Niagara Falls, N. Y., will supply
one.)
2. A small crystal of sapphire (not of gem quality, but it should be
transparent and compact. A pale or colorless Montana sapphire can be had
for a few cents of any mineral dealer).
3. A small _true topaz_ crystal. (The pure white topaz of Thomas
Mountain, Utah, is excellent; or white topaz from Brazil or Japan or
Mexico or Colorado will do. Any mineral house can furnish small crystals
for a few cents when not of specially fine crystallization.)
4. A small quartz crystal. (This may be either amethyst or quartz-topaz
or the common colorless variety. The fine, sharp, colorless crystals
from Herkimer County, N. Y., are excellent. These are very inexpensive.)
5. A fragment of a crystal of feldspar. (Common orthoclase feldspar,
which is frequently of a brownish pink or flesh color, will do.)
These five test stones represent the following degrees of hardness:
1. Carborundum is harder than any gem material but diamond. It will
scratch sapphire and ruby, which are rated 9 in hardness, hence we may
call carborundum 9-1/2 if we wish. It is, however, very much softer than
diamond, and the latter will scratch it upon the slightest pressure.
2. Sapphire, of hardness 9, scratching any gem material except diamond.
3. True topaz, of hardness 8. It is scratched by sapphire (and, of
course, ruby), also by chrysoberyl (which is hence rated 8-1/2), but
scratches most other stones. Spinel (which is also rated as 8 in
hardness) is really a bit harder than topaz.
4. Quartz, of hardness 7, and scratched by all the previous stones but
scratching those that were listed above as of less hardness than a file.
5. Feldspar, of hardness 6, hence slightly softer than a file and
yielding to it, but scratching the stones likewise rated as 6 when
applied forcibly to them. Also scratching stones rated as less than 6 on
slight pressure.
We must next consider how these minerals may be safely used upon gem
material. Obviously it would be far safer to use them upon rough gem
material than upon cut stones. However, with care and some little skill,
one may make hardness tests without particular danger to fine cut
material.
The way to proceed is to apply the cut stone (preferably its girdle, or
if that is so set as not to be available, a corner where several facets
meet) gently to the flat surface of one of the softer test stones,
drawing it lightly along the surface and noting the _feel_ and looking
to see if a scratch results. If the test stone is scratched try the next
harder test stone similarly. _Do not attempt to use the test stone upon
any valuable cut stone._ Proceed as above until the gem meets a test
stone that it does not attack. Its hardness is then probably equal to
the latter and perhaps if pressed forcibly against it a slight scratch
would result, but it is not advisable to resort to heavy pressure. A
light touch should be cultivated in this work. Having now an indication
as to the hardness of the unknown gem look up in the table of the
previous lesson those gems of similar hardness and then by the use of
some of the tests already given decide which of the stones of that
degree of hardness you have. _Never rely upon a single test in
identifying a gem._
For further study of hardness and its use in testing gems see
_Gem-Stones_, G. F. Herbert-Smith, Chap. IX., pp. 78-81, and table on p.
305; or see _A Handbook of Precious Stones_, Rothschild, pp. 19, 20,
21.
LESSON X
DISPERSION
Another property which may be made use of in deciding the identity of
certain gems is that called _dispersion_. We have seen in Lesson II.
that light in entering a stone from the air changes its path
(refraction), and in Lesson III. it was explained that many minerals
cause light that enters them, to divide and proceed along two different
paths (double refraction). Now it is further true that light of the
various colors (red, orange, yellow, green, blue, and violet) is
refracted variously--the violet being bent most sharply, the red least,
and the other colors to intermediate degrees. The cut (Fig. 7)
represents roughly and in an exaggerated manner the effect we are
discussing.
[Illustration: FIG. 7.]
Now in a cut stone this separation of light of different colors, or
dispersion of light, as it is called, results in the reflection of each
of the colors separately from the steep sloping back facets of the
stone. If almost any clear, colorless facetted stone is placed in the
sunlight and a card held before it to receive the reflections, it will
be seen that rainbow-like reflections appear on the card. These
_spectra_, as they are called, are caused by the dispersion of light.
With a diamond the spectra will be very brilliant and of vivid coloring,
and the red will be widely separated from the blue. With white sapphire
or white topaz, or with rock crystal (quartz), the spectra will be less
vivid--they will appear in pairs (due to the double refraction of these
minerals), and the red and blue will be near together (_i. e._, the
spectra will be short). This shortness in the latter cases is due to the
small dispersive power of the three minerals mentioned. Paste (lead
glass) gives fairly vivid spectra, and they are single like those from
diamond, as glass is singly refracting. The dispersion of the heavy lead
glass approaches that of diamond. The decolorized zircon (jargoon) has a
dispersion well up toward that of diamond and gives fairly vivid spectra
on a card, but they are double, as zircon is doubly refracting. Sphene
(a gem rarely seen in the trade) and the demantoid garnet (a green gem
often called "olivine" in the trade) both have very high dispersive
power, exceeding the diamond in this respect. As they are both colored
stones (sphene is usually yellowish, sometimes greenish or brown), the
vividness of their color-play is much diminished by absorption of light
within them. So also the color-play of a deeply colored fancy diamond
is diminished by absorption.
DISPERSION AS A TEST OF THE IDENTITY OF A GEM. We may now consider how
an acquaintance with the dispersive powers of the various stones can be
used in distinguishing them. If a stone has high dispersive power it
will exhibit "fire," as it is called--_i. e._, the various colors will
be so widely separated within the stone, and hence reflected out so
widely separated, that they will fall on the eye (as on the card above)
in separate layers, and vivid flashes of red or yellow or other colors
will be seen. Such stones as the white sapphire (and others of small
dispersion), however, while separating the various colors appreciably as
seen reflected on a card, do not sufficiently separate them to produce
the "fire" effect when the light falls on the eye. This is because the
various colors, being very near together in this case, cross the eye so
rapidly, when the stone is moved, that they blend their effect and the
eye regards the light that thus falls upon it as white. We have here a
ready means of distinguishing the diamond from most other colorless
gems. The trained diamond expert relies (probably unconsciously) upon
the dispersive effect (or "fire") nearly as much as upon the adamantine
luster, in telling at a glance whether a stone is or is not a diamond.
Of all colorless stones, the only one likely to mislead the expert in
this respect is the whitened zircon (jargoon), which has almost
adamantine luster and in addition nearly as high dispersive power as
diamond. However, zircon is doubly refracting (strongly so), and the
division of the spectra which results (each facet producing two instead
of only one) weakens the "fire" so that even the best zircon is a bit
"sleepy" as compared with even an ordinary diamond.
In addition to providing a ready means of identifying the diamond, a
high degree of dispersion in a stone of pronounced color would lead one
to consider sphene, demantoid garnet (if green), and zircon (which
might be reddish, yellowish, brown, or of other colors), and if the
stone did not agree with these in its other properties one should
suspect _glass_.
A good way to note the degree of dispersion, aside from the
sunlight-card method, is to look at the stone from the back while
holding it up to the light (daylight). Stones of high dispersive power
will display vivid color play in this position. Glass imitations of
rubies, emeralds, amethysts, etc., will display altogether too much
dispersion for the natural gems.
In Chap. III., p. 20, of G. F. Herbert-Smith's _Gem-Stones_, a brief
account of dispersion is given. College text-books on physics also treat
of it, and the latter give an account of how dispersion is measured and
what is meant by a coefficient of dispersion. Most gem books say little
about it, but as we have seen above, a knowledge of the matter can, when
supplemented by other tests, be applied practically in distinguishing
gems.
LESSON XI
COLOR
In reserving to the last the property of _color_, which many dealers in
gems use first when attempting to identify a precious stone, I have
sought to point out the fact that a determination based solely upon
color is very likely to be wrong. So many mineral species are found in
so many different colors that to attempt to identify any mineral species
by color alone is usually to invite disaster. The emerald, alone among
gems, has, when of fine color, a hue that is not approached by any other
species. The color of the grass in the springtime fitly describes it.
Yet even here the art of man has so closely counterfeited in glass the
green of the emerald that one cannot be sure of his stone by color
alone. As was suggested earlier in these lessons, the writer has
several times recently had occasion to condemn as glass imitations
stones for which high prices had been paid as genuine emeralds, those
who sold them having relied solely upon a trained eye for color.
CONFUSION OF GEMS DUE TO SIMILARITY OF COLOR. The same tendency to rely
upon color causes many in the trade to call all yellow stones "topaz"
whether the species be corundum (oriental topaz), true topaz (precious
topaz), citrine quartz (quartz topaz), heliodor (yellow beryl), jacinth
(yellow zircon), or what not.
Similarly the public calls all red stones ruby. Thus we have "cape ruby"
and "Arizona ruby" (pyrope garnet), "spinel ruby" (more properly ruby
spinel), "Siam ruby" (very dark red corundum), "Ceylon ruby" (pale
pinkish corundum), rubellite (pink tourmaline), and lastly Burmah ruby
(the fine blood-red corundum).
While it is true that color, unless skillfully estimated and wisely used
in conjunction with other properties, is a most unreliable guide, yet
when thus used, it becomes a great help and serves sometimes to narrow
down the chase, at the start, to a very few species. To thus make use of
it requires an actual acquaintance with the various gem materials, in
their usual colors and shades and an eye trained to note and to remember
minute differences of tint and shade. The suggestions which follow as to
usual colors of mineral species must then be used only with discretion
and after much faithful study of many specimens of each of the species.
Let us begin with the beginning color of the visible spectrum, red, and
consider how a close study of shades of red can help in distinguishing
the various red stones from each other. In the first place we will
inquire what mineral species are likely to furnish us with red stones.
Omitting a number of rare minerals, we have (1) corundum ruby, (2)
garnet of various types, (3) zircon, (4) spinel, (5) tourmaline. These
five minerals are about the only common species which give us an
out-and-out red stone. Let us now consider the distinctions between the
reds of these different species. The red of the ruby, whether dark (Siam
type), blood red (Burmah type), or pale (Ceylon), is more pleasing
usually than the red of any of the other species. Viewed from the back
of the stone (by transmitted light) it is still pleasing. It may be
purplish, but is seldom orange red. Also, owing to the dichroism of the
ruby the red is variable according to the changing position of the
stone. It therefore has a certain life and variety not seen in any of
the others except perhaps in red tourmaline, which, however, does not
approach ruby in fineness of red color.
RED STONES OF SIMILAR SHADES. The garnet, on the other hand, when of
fire-red hue, is darker than any but the Siam ruby. It is also more
inclined to orange red or brownish red--and the latter is especially
true when the stone is seen against the light (by transmitted light).
Its color then resembles that of a solution of "iron" such as is given
as medicine. The so-called "almandine" garnets (those of purplish-red
tint) do not equal the true ruby in brightness of color and when held up
to the light show more prismatic colors than the true ruby, owing to the
greater dispersion of garnet. The color also lacks variety (owing to
lack of dichroism). While a fine garnet may make a fair-looking "ruby"
when by itself, it looks inferior and dark when beside a fine ruby. By
artificial light, too, the garnet is dark as compared with the true
ruby, and the latter shows its color at a distance much more strongly
than the garnet.
The red zircon, or true hyacinth, is rare. (Many hessonite garnets are
sold as hyacinths in the trade. These are usually of a brownish red.)
The red of the hyacinth is never equal to that of the ruby. It is
usually more somber, and a bit inclined to a brownish cast. The
dispersion of zircon, too, is so large (about 87 per cent. of that of
diamond) that some little "color-play" is likely to appear along with
the intrinsic color. The luster too is almost adamantine while that of
ruby is softer and vitreous. Although strongly doubly refracting, the
hyacinth shows scarcely any dichroism and thus lacks variety of color.
Hence a trained eye will at once note these differences and not confound
the stone with ruby.
Spinels, when red, are almost always more yellowish or more purplish
than fine corundum rubies. They are also singly refracting and hence
exhibit no dichroism and therefore lack variety of color as compared
with true ruby. Some especially fine ones, however, are of a good enough
red to deceive even jewelers of experience, and one in particular that I
have in mind has been the rounds of the stores and has never been
pronounced a spinel, although several "experts" have insisted that it
was a scientific ruby. The use of a dichroscope would have saved them
that error, for the stone is singly refracting. Spinels are usually
clearer and more transparent than garnets and show their color better at
a distance or when in a poor light.
Tourmaline of the reddish variety (rubellite) is seldom of a deep red.
It is more inclined to be pinkish. The dichroism of tourmaline is
stronger than that of ruby and more obvious to the unaided eye. The red
of the rubellite should not deceive anyone who has ever seen a fine
corundum ruby.
YELLOW STONES
Considering next the stones of yellow color, we have the following
species to deal with: (1) diamond, (2) corundum, (3) precious topaz, (4)
quartz, (5) beryl, (6) zircon, (7) tourmaline.
YELLOW ZIRCON RESEMBLES YELLOW DIAMOND. Here we have less opportunity to
judge of the species by the color than was the case with the red
stones. The diamond, of course, is easy to tell, not by the kind of
yellow that it displays, for it varies greatly in that respect, but
rather by its prismatic play blended with the intrinsic color. Its
luster also gives an immediate clue to its identity. It is necessary,
however, to be sure that we are not being deceived by a yellow zircon,
for the latter has considerable "fire" and a keen luster. Its strong
double refraction and its relative softness, as well as its great
density will serve to distinguish it. Of the other yellow stones, the
true or precious topaz is frequently inclined to a pinkish or wine
yellow and many such stones lose all their yellow (retaining their pink)
when gently heated. The so-called "pinked" topazes are thus produced.
The yellow corundum rarely has a color that is at all distinctive. As
far as color goes the material might be yellow quartz, or yellow beryl,
or yellow zircon, or yellow tourmaline (Ceylon type). Many of the
yellowish tourmalines have a decidedly greenish cast (greenish-yellow
chrysoberyl might resemble these also). However, in general if one has a
yellow stone to determine it will be safer to make specific gravity or
hardness tests, or both, before deciding, rather than to rely upon
color.
LESSON XII
COLOR--_Continued_
GREEN STONES
Let us first consider what mineral species are most likely to give us
green stones. Omitting the semi-precious opaque or translucent stones we
have:
1. Grass-green beryl (the emerald) which is, of course, first in
value among the green stones and first in the fine quality of its
color.
2. Tourmaline (some specimens of which perhaps more nearly approach
the emerald than any other green stones).
3. The demantoid garnet (sometimes called "olivine" in the trade).
4. True olivine (the peridot and the chrysolite of the trade).
5. Bluish-green beryl (aquamarine).
6. Green sapphire (Oriental emerald or Oriental aquamarine).
7. Chrysoberyl (alexandrite and also the greenish-yellow
chrysoberyl).
1. Considering first the emerald, we have as legitimate a use of color
in distinguishing a stone as could be selected, for emerald of fine
grass-green color is not equaled by any other precious stone in the rich
velvety character of its color. We have to beware here, however, of the
fine glass imitations, which, while lacking the variety of true emerald,
because of lack of dichroism, are nevertheless of a color so nearly like
that of the emerald that no one should attempt to decide by color alone
as to whether a stone is genuine or imitation emerald. If a hardness
test shows that the material is a genuine hard stone and not a paste,
then one who is well accustomed to the color of fine emerald can say at
once whether a stone is a fine emerald or some other hard green stone.
Where the color is less fine, however, one might well refuse to decide
by the color, even when sure that the material is not glass, for some
fine tourmalines approach some of the poorer emeralds in richness of
color.
THE "SCIENTIFIC EMERALD" FRAUD. No "scientific" emeralds of marketable
size have ever been produced as far as can be learned. Many attempts to
reproduce emerald by melting beryl or emerald of inferior color have
resulted only in the production of a beryl glass, which, while its color
might be of desirable shade, was softer and lighter in weight than true
emerald. It was also a true glass and hence singly refracting and
without dichroism, whereas emerald is crystalline (not glassy or
amorphous), is doubly refracting, and shows dichroism.
Do not be misled, then, into buying or selling an imitation of emerald
under the terms "synthetic," "scientific," or "reconstructed," as such
terms, when so used, are used to deceive one into thinking that the
product offered bears the same relation to the true emerald that
scientific rubies and sapphires bear to the natural stones. Such is not
the case.
About the most dangerous imitation of the emerald that is ever seen in
the trade is the triplet that has a top and a back made of true but pale
beryl (the same mineral as emerald, but not of the right color) and a
thin slice of deep emerald green glass laid between. This slice of glass
is usually placed behind the girdle so that a file will not find any
point of attack. The specific gravity of the triplet is practically that
of emerald, its color is often very good, and it is doubly refracting.
It is thus a dangerous imitation. (See Fig. 8.)
EMERALD TRIPLETS. A careful examination of one of these triplets, in the
unset condition, with a good lens, will reveal the thin line of junction
of the beryl with the glass. (The surface lusters of the two materials
are enough different for the trained eye to detect the margin at once.)
Such a triplet, if held in the sun, will reflect onto a card two images
in pale or white light, one coming from the top surface of the table and
the other from the top surface of the glass slice within. In other
words, it acts in this respect like a doublet. A true emerald would give
only one such reflection, which would come from the top surface of the
table.
[Illustration: FIG. 8.--EMERALD TRIPLET.]
2. Tourmalines, when green, are usually darker than emeralds and of a
more pronounced yellow green, or they may be of too bluish a green, as
is the case with some of the finest of the green tourmalines from
Maine. Connecticut green tourmaline tends more to the dark yellowish
green, and Ceylon tourmaline to the olive green. The stronger dichroism
of the tourmaline frequently reveals itself to the naked eye, and there
is usually one direction or position in which the color of the stone is
very inferior to its color in the opposite direction or position. Most
tourmalines (except the very lightest shades) must be cut so that the
table of the finished stone lies on the side of the crystal, as, when
cut with the table lying across the crystal (perpendicular to the
principal optical axis) the stones are much too dark to be pretty. Hence
when one turns the cut stone so that he is looking in the direction
which was originally up and down the crystal (the direction of single
refraction and of no dichroism) he gets a glimpse of a less lovely color
than is furnished by the stone in other positions. With a true emerald
no such disparity in the color would appear. There might be a slight
change of shade (as seen by the naked eye), but no trace of an ugly
shade would appear.
By studying many tourmalines and a few emeralds one may acquire an eye
for the differences of color that characterize the two stones, but it is
still necessary to beware of the fine glass imitation and to use the
file and also to look with a high-power glass for any rounding bubbles.
The emerald will never have the latter. The glass imitation frequently
does have them. The sharp jagged flaws and cracks that so often appear
in emerald are likely to appear also in tourmaline as both are brittle
materials. The glass imitations frequently have such flaws put into them
either by pinching or by striking the material. Frequently, too, wisps
of tiny air bubbles are left in the glass imitations in such fashion
that unless one scrutinizes them carefully with a good lens they
strongly resemble the flaws in natural emerald.
I have thus gone into detail as to how one may distinguish true emerald
from tourmaline and from glass imitations because, on account of the
high value of fine emerald and its infrequent occurrence, there is
perhaps more need for the ability to discriminate between it and its
imitations and substitutes than there is in almost any other case. Where
values are high the temptation to devise and to sell imitations or
substitutes is great and the need for skill in distinguishing between
the real and the false is proportionally great.
3. The demantoid garnet (often unfortunately and incorrectly called
"olivine" in the trade) is usually of an olive or pistachio shade. It
may, however, approach a pale emerald. The refraction being single in
this, as in all garnets, there is little variety to the color. The
dispersion being very high, however, there is a strong tendency, in
spite of the depth of the body color, for this stone to display "fire,"
that is, rainbow color effects. The luster, too, is diamond-like as the
name "demantoid" signifies. With this account of the stone and a few
chances to see the real demantoid garnet beside an emerald no one would
be likely to mistake one for the other. The demantoid garnet is also
very soft as compared with emerald (6-1/2 as against nearly 8).
4. True olivine (the peridot or the chrysolite of the trade) is of a
fine leaf-green or bottle-green shade in the peridot. The chrysolite of
the jeweler is usually of a yellower green. Frequently an olive-green
shade is seen. The luster of olivine (whether of the peridot shade or
not) is oily, and this may serve to distinguish it from tourmaline
(which it may resemble in color). Its double refraction is very large
also, so that the doubling of the edges of the rear facets may easily be
seen through the table with a lens. The dichroism is feeble too, whereas
that of tourmaline is strong. No one would be likely to confuse the
stone with true emerald after studying what has preceded.
5. Bluish-green beryl (aquamarine) is usually of a pale transparent
green or blue green (almost a pure pale blue is also found).
Having all the properties of its more valuable variety, emerald, the
pale beryl may, by the use of these properties, be distinguished from
the pale blue-green topaz which so strongly resembles it in color.
6. Green sapphire seldom even approaches emerald in fineness of color.
When it remotely suggests emerald it is called "Oriental" emerald to
denote that it is a corundum gem. Most green sapphires are of too blue a
green to resemble emerald. Some are really "Oriental" aquamarines. In
some cases the green of the green sapphire is due to the presence,
within the cut stone, of both blue and yellow portions, the light from
which, being blended by its reflection within the stone, emerges as
green as seen by the unaided eye, which cannot analyze colors. The dark
sapphires of Australia are frequently green when cut in one direction
and deep blue when cut in the opposite direction. The green, however,
is seldom pleasing.
7. Chrysoberyl as usually seen is of a yellowish green. The fine gem
chrysoberyls known as alexandrites, however, have a pleasing bluish
green or deep olive green color by daylight and change in a most
surprising fashion by artificial light under which they show raspberry
red tints. This change, according to G. F. Herbert-Smith, is due
principally to the fact that the balance in the spectrum of light
transmitted by the stone is so delicate that when a light, rich in short
wave lengths, falls upon it the blue green effect is evident, whereas
when the light is rich in long wave lengths (red end of the spectrum),
the whole stone appears red. The strong dichroism of the species also
aids this contrast. The chrysoberyls of the cat's-eye type (of fibrous
or tubular internal structure) are usually of olive green or
brownish-green shades.
Those who wish to further study color distinctions in green stones are
recommended to see the chapters on beryl (pp. 184-196), peridot (pp.
225-227), corundum (pp. 172-183), tourmaline (pp. 219-224), chrysoberyl
(pp. 233-237), and garnet (demantoid, pp. 216-218) in G. F.
Herbert-Smith's _Gem-Stones_.
LESSON XIII
COLOR--_Continued_
BLUE STONES
The species that furnish blue stones in sufficient number to deserve
consideration are, aside from opaque stones:
1. Corundum (sapphire).
2. Spinel.
3. Tourmaline.
4. Topaz.
5. Diamond.
6. Zircon.
1. Of these minerals the only species that furnishes a fine, deep
velvety blue stone is the corundum, and fine specimens of the cornflower
blue variety are very much in demand and command high prices. The color
in sapphires ranges from a pale watery blue through deeper shades (often
tinged with green) to the rich velvety cornflower blue that is so much
in demand, and on to dark inky blues that seem almost black by
artificial light. Most sapphires are better daylight stones than evening
stones. Some of the sapphires from Montana, however, are of a bright
electric blue that is very striking and brilliant by artificial light.
HOW SAPPHIRES SHOULD BE CUT. The direction in which the stone is cut
helps determine the quality of the blue color, as the "ordinary" ray
(sapphire exhibits dichroism) is yellowish and ugly in color, and if
allowed to be conspicuous in the cut stone, its presence, blending with
the blue, may give it an undesirable greenish cast. Sapphires should
usually be cut so that the table of the finished stone is perpendicular
to the principal optical axis of the crystal. Another way of expressing
this fact is that the table should cross the long axis of the usual
hexagonal crystal of sapphire, at right angles. This scheme of cutting
puts the direction of single refraction up and down the finished stone,
and leaves the ugly ordinary rays in poor position to emerge as the
light that falls upon the girdle edges cannot enter and cross the stone
to any extent.
To find out with a finished stone whether or not the lapidary has cut it
properly as regards its optical properties one may use the dichroscope,
and if there is little or no dichroism in evidence when looking through
the table of the stone it is properly cut.
Where a sapphire shows a poor color and the dichroscope shows that the
table was laid improperly, there is some possibility of improving the
color by recutting to the above indicated position. However, one must
use much judgment in such a case, as sapphires, like other corundum
gems, frequently have their color irregularly distributed, and the
skillful lapidary will place the culet of the stone in a bit of good
color, and thus make the whole stone appear to better advantage. It
would not do to alter such an arrangement, as one would get poorer
rather than better color by recutting in such a case.
While some of the blue stones about to be described may resemble
inferior sapphires, none of them approaches the better grades of
sapphire in fineness of blue coloration. The scientific sapphire, of
course, does approach and even equals the natural sapphire so that one
must know how to distinguish between them. This distinction is not one
of color, however, and it will be separately considered a little later.
2. Blue spinels are infrequently seen in commerce. They never equal the
fine sapphire in their color, being more steely. They, of course, lack
dichroism and are softer than sapphire as well as lighter.
3. Blue tourmalines are never of fine sapphire blue. The name indicolite
which mineralogists give to these blue stones suggests the indigo-blue
color which they afford. The marked dichroism of tourmaline will also
help detect it. Some tourmalines from Brazil are of a lighter shade of
blue and are sometimes called "Brazilian sapphires."
4. Blue topaz is usually of a pale sky blue or greenish blue and is
likely to be confused with beryl of similar color. The high density of
topaz (3.53) as compared with beryl (2.74) serves best to distinguish
it.
"FANCY" BLUE DIAMONDS. 5. Blue diamonds are usually of very pale bluish
or violet tint. A few deeper blue stones are seen occasionally as
"fancy" diamonds. These are seldom as deep blue as pale sapphires. Even
the famous Hope Blue Diamond, a stone of about forty-four carats and of
great value, is said to be too light in color to be considered a fine
sapphire blue. Some of the deeper blue diamonds have a steely cast. The
so-called blue-white stones are rarely blue in their body color, but
rather are so nearly white that the blue parts of the spectra which they
produce are very much in evidence, thus causing them to face up blue.
There is little likelihood of mistaking a bluish diamond for any other
stone on account of the "fire" and the adamantine luster of the diamond.
6. Blue zircon, however, has nearly adamantine luster and considerable
fire. The color is usually sky blue. Such stones are seldom met with in
the trade.
For a more detailed account of the various blue stones see G. F.
Herbert-Smith's _Gem-Stones_, as follows:
For sapphires, pp. 172-173, 176, 182; for spinel, pp. 203, 204, 205; for
tourmaline, pp. 220, 221, 223; for topaz, pp. 198, 200, 201; for
diamond, pp. 130, 136, 170, and for zircon, pp. 229, 231.
LESSON XIV
COLOR--_Concluded_
PINK, PURPLE, BROWN, AND COLORLESS STONES
PINK STONES. Pink stones are yielded by (1) corundum (pink sapphire),
(2) spinel (balas ruby), (3) tourmaline (rubellite), (4) true topaz
(almost always artificially altered), (5) beryl (morganite), (6)
spodumene (kunzite), and (7) quartz (rose-quartz).
These pink minerals are not easily differentiated by color alone, as the
depth and quality of the pink vary greatly in different specimens of the
same mineral and in the different minerals. There is dichroism in the
cases of pink sapphire, pink tourmaline (strong), pink topaz (strong),
pink beryl (less pronounced), and kunzite (very marked and with a
yellowish tint in some directions that contrasts with the beautiful
violet tint in another direction in the crystal). Pink quartz is almost
always milky, and shows little dichroism. Pink spinel is without
dichroism, being singly refracting. Hardness and specific gravity tests
will best serve to distinguish pink stones from each other. The color
alone is not a safe guide.
PURPLE STONES. Among the mineral species that furnish purple stones, (1)
quartz is pre-eminent in the fineness of the purple color. Such purple
stones are, of course, known as amethysts. After quartz come (2)
corundum (Oriental amethyst), (3) spinel (almandine spinel), (4) garnet
(almandine), and (5) spodumene (variety kunzite).
The purple of the amethyst varies from the palest tints to the full rich
velvety grape-purple of the so-called Siberian amethysts. The latter are
of a reddish purple (sometimes almost red) by artificial light, but of a
fine violet by daylight. No other purple stone approaches them in
fineness of coloring, so that here we have a real distinction based on
color alone. If the purple is paler, however, one cannot be sure of the
mineral by its color. Purple corundum (Oriental amethyst) is seldom as
fine in color as ordinary amethyst, and never as fine as the best
amethyst. It is usually of a redder purple, and by artificial light is
almost ruby-like in its color.
Purple spinels are singly refracting, and lack dichroism, and hence lack
variety of color. Almandine garnets also show no dichroism and lack
variety of color. The garnets are, as a rule, apt to be more dense in
color than the spinels.
Purple spodumene (kunzite) is pinkish to lilac in shade--usually pale,
unless in large masses, and it shows very marked dichroism. A yellowish
cast of color may be seen in certain directions in it also, which will
aid in distinguishing it from other purple stones.
BROWN STONES. (1) Diamond, (2) garnet, (3) tourmaline, and (4) zircon
furnish the principal brown stones.
Diamond, when brown, unless of a deep and pleasing color, is very
undesirable, as it absorbs much light, and appears dirty by daylight and
dark and sleepy by artificial light. When of a fine golden brown a
diamond may have considerable value as a "fancy" stone. Such "golden
fancies" can be distinguished from other brown stones (except perhaps
brown zircons) by their adamantine luster, and their prismatic play or
"fire."
Brown garnet (hessonite or cinnamon stone), sometimes wrongly called
hyacinth in the trade, is of a deep reddish-brown color. Usually the
interior structure, as seen under a lens, is streaky, having a sort of
mixed oil and water appearance.
Brown tourmaline is sometimes very pleasing in color. It is deep in
shade, less red than cinnamon stone, and with marked dichroism, which
both brown diamond and brown garnet lack.
Brown zircon, while lacking dichroism, is frequently rich and pleasing
in shade, and when well cut is very snappy, the luster being almost
adamantine, the dispersion being large, and the refractive index high.
It is useless to deny that by the unaided eye one might be deceived into
thinking that a fine brown zircon was a brown diamond. However, the
large double refraction of the zircon easily distinguishes it from
diamond (use the sunlight-card method or look for the doubling of the
edges of the rear facets as seen through the table). The relative
softness (7-1/2) also easily differentiates it from diamond.
COLORLESS STONES. Few colorless stones other than diamond, white
sapphire (chiefly scientific), and quartz are seen in the trade.
Colorless true topaz is sometimes sold and artificially whitened zircon
(jargoon) is also occasionally met with. Beryl of very light green tint
or even entirely colorless may also be seen at times.
Such colorless stones must of course be distinguished by properties
other than color. They are mentioned here merely that the learner may be
aware of what varieties of gem minerals occur in the colorless
condition, and that all these minerals also occur with color in their
more usual forms. This does not even except the diamond, which is rarely
truly colorless.
LESSON XV
HOW TO TELL SCIENTIFIC STONES FROM NATURAL GEMS
It should be said first that the only true scientific or synthetic
stones on the market are those having the composition and properties of
corundum, that is to say, the ruby and the several color varieties of
sapphire, as blue, pink, yellow, and white. There is also a greenish
stone that appears reddish by artificial light, which is called
scientific alexandrite but which has, however, the composition and
properties of the corundum gems rather than those of true alexandrite.
All so-called "scientific emeralds" have proved to be either of paste of
one sort or another, or else triplets having a top and a back of some
inexpensive but hard stone of pale color, and a central slice of deep
green glass, the three pieces being cemented together so skillfully
that the junctions frequently escape any but a very careful examination
with a lens.
ALL SCIENTIFIC STONES ARE CORUNDUM GEMS. Now the fact that all true
scientific stones are corundum gems makes their determination fairly
simple on the following basis: Among the considerable number of corundum
gems of nature, whether ruby or sapphire of various colors, there is
seldom found one that is entirely free from defects. Almost always, even
in what are regarded as fine specimens, one will easily find with a
glass, defects in the crystallization. Moreover these defects are
characteristic of the corundum gems.
The scientific corundum gems, however, never have these specific
defects. Hence the surest and simplest way of distinguishing between the
two kinds of stones is to acquaint oneself with the typical defects of
natural corundum gems, and then to look for such defects in any specimen
of ruby or sapphire that is in question.
While a description of some of the most common of the typical defects of
rubies and sapphires is to follow, the jeweler, who may not yet be
familiar with them by actual experience, owes it to himself and to his
customers to acquaint himself at first hand with the natural defects of
such material, which he is always in a position to do through the
courtesy of representatives of houses dealing in precious stones, if he
himself does not carry such material in stock.
TYPICAL DEFECTS OF NATURAL CORUNDUM GEMS. Perhaps the most common of the
defects of natural corundum gems is the peculiar appearance known as
"silk." This is best seen when a strong light is allowed to stream
through the stone at right angles to the observer's line of sight. Sets
of fine, _straight_, parallel lines will be seen, and these will
frequently meet other sets of similar lines at an angle of 120 degrees
(like the angle at which the sides of a regular hexagon meet) or the
lines may cross each other at that angle or at an angle of 60 degrees
(the supplement of 120 degrees). Such _straight_ parallel lines are
never seen in scientific stones, and their presence may be taken to
indicate positively that the stone having them is a natural stone. In
fine specimens of natural ruby or sapphire such lines will be few and
difficult to find, but in some position or other they will usually be
found if the search is even as careful as that which one would
habitually employ in looking for defects in a diamond. In the vast
majority of cases no such careful search will be required to locate
"silk" in natural rubies, and if a stone that is apparently a ruby is
free from such defects it is almost a foregone conclusion it is a
scientific stone.
Another common type of defect in corundum gems is the occurrence of
patches of milky cloudiness within the material. A little actual
acquaintance with the appearance of this sort of defect in natural
stones will make it easy to distinguish from the occasional cloudiness
found in scientific stones, which latter cloudiness is due to the
presence of swarms of minute gas bubbles. These tiny bubbles can be seen
under a high power lens, and this suggests a third feature that may be
used to tell whether one has a natural stone or not.
Natural rubies and sapphires, like scientific ones, frequently contain
bubbles, but these are always _angular_ in the natural stones, while
those of the scientific stones are generally _round_ or rounding, never
angular.
To sum up the suggestions already presented it may be said that, since
natural and scientific corundum gems are composed of essentially the
same material, and have identically the same physical and chemical
properties, and frequently very closely resemble each other in color, it
is necessary to have recourse to some other means of distinguishing
between them. The best and simplest means for those who are acquainted
with the structural defects common to natural corundum gems is to seek
for such defects in any specimen that is in question, and if no such
defects can be found, to be very sceptical as to the naturalness of the
specimen, inasmuch as perfect corundum gems are very rare in nature, and
when of fine color command exceedingly high prices. No jeweler can
afford to risk his reputation for knowledge and for integrity by selling
as a natural stone any gem which does not possess the minor defects
common to practically all corundum gems.
STRUCTURAL DEFECTS OF SCIENTIFIC STONES. So far our tests have been
mostly negative. It was said, however, that spherical bubbles sometimes
appear in scientific gems. Another characteristic _structural_ defect of
practically every scientific gem may be utilized to distinguish them. As
is well known, the rough material is formed in boules or pear-shaped
drops under an inverted blowpipe. The powdered material is fed in with
one of the gases and passes through the flame, melting as it goes, and
then accumulating and crystallizing below as a boule. The top or head
of this boule is rounding from the start, and hence the successive
layers of material gather in thin curved zones. The color and structure
of these successive zones are not perfectly uniform, hence when cut
stones are made from the boules these _curving_ parallel layers may be
seen within by the use of a good lens, especially if the cut stone is
held in a strong crossing light, as was suggested when directions were
given above as to the best way to look for "silk" in a natural stone.
Owing to the shape of a well cut stone it is sometimes difficult to get
light through the material, yet by turning the stone repeatedly, some
position will be found in which the curving parallel striæ can be seen.
They are easily seen in scientific ruby, less easily in dark blue
sapphire, but still they can be found on close search. In the light
colored stones and in white sapphire, the difficulty is greater, as
there are no color variations in the latter case. However, the value of
white sapphire is so slight, whether natural or artificial, that it is a
matter of but little moment, and what has already been said as to
natural defects, applies to white sapphire as well as to the colored
varieties, and absolutely clear and perfect natural white sapphire is
rare.
One more distinguishing mark of the scientific stones may be added to
give full measure to the scheme of separation, that no one need be
deceived.
The surface finish of the scientific stones is rarely as good as that of
the natural material and it appears to be more difficult to produce a
good polish on scientific stones than on natural ones. The degree of
hardness of the scientific stones seems to be slightly variable in
different parts of the same piece so that the polishing material removes
the surface material unequally, leaving minute streaky marks on the
surfaces of the facets. Possibly this condition might be remedied by
skillful treatment, but hardly at the price obtainable for the product,
so that a close study of the surface finish will sometimes help in
distinguishing between natural and artificial material. Any fine
specimen of natural ruby or sapphire will have usually received very
expert treatment and a splendid surface finish.
In conclusion, then, the points to be remembered in determining the
origin of corundum gems are four in number.
1. Expect to find natural defects, such as "silk" or cloudy patches, or
_angular_ bubbles in all natural stones.
2. If bubbles are present in artificial material they will be _round_ or
rounding.
3. Artificial material will always have _curving_ parallel striæ within
it.
4. The _surface finish_ of artificial material is seldom or never equal
to that of natural material.
It ought not to be necessary to add that material from either source may
be cut to any shape, and that artificial rubies may be seen in most
Oriental garb, hence all specimens should have applied to them the above
tests regardless of the seeming antiquity of their cut or of their
alleged pedigree.
LESSON XVI
HOW TO TEST AN "UNKNOWN" GEM
Having now considered separately the principal physical properties by
means of which one can identify a precious stone, let us attempt to give
as good an idea as the printed page can convey of how one should go
about determining to what species a gem belongs.
SIGNS OF WEAR IN AN EMERALD. To make the matter more concrete, and
therefore more interesting, let us consider a real case, the most recent
problem, in fact, that the author has had to solve. A lady of some
wealth had purchased, for a large sum, a green stone which purported to
be an emerald. After a few years of wear as a ring stone she noticed one
day that the stone had dulled around the edges of its table, and
thinking that that ought not to be the case with a real emerald, she
appealed to a dealer in diamonds to know if her stone was a real
emerald. The diamond merchant told her frankly that, while he was
competent in all matters pertaining to diamonds, he could not be sure of
himself regarding colored stones, and advised the lady to see the
author.
The matter being thus introduced, the lady was at once informed that
even a real emerald might show signs of wear after a few years of the
hard use that comes to a ring stone.
While emerald has, as we saw in the lesson on hardness, a degree of
hardness rated as nearly 8 (7-1/2 in the table), it is nevertheless a
rather brittle material and the long series of tiny blows that a ring
stone is bound to meet with will cause minute yielding along the exposed
edges and corners of the top facets. This being announced, the first
step in the examination of the stone was to clean it and to give it a
careful examination with a ten-power lens. (An aplanatic triplet will
be found best for this purpose.)
COLOR. The color was, of course, the most obvious property, but, as has
already been said, color is not to be relied upon in all cases. In this
case the color was a good emerald green but a bit bluer than the finest
grass green. A very fine Maine tourmaline might approach this stone in
color, so it became necessary to consider this possibility. A glass
imitation, too, might have a color equal or superior to this.
IMPERFECTIONS. While noting the color, the imperfections of the stone
claimed attention. They consisted mainly of minute jagged cracks of the
character peculiar to brittle materials such as both emerald and
tourmaline. So far it will be noted either of the above minerals might
have furnished the lady's gem. As glass can be artificially crackled to
produce similar flaws the stone might have been only an imitation as far
as anything yet learned about it goes.
FILE TEST. The next step was to test its hardness by gently applying a
very fine file to an exposed point at one corner of the girdle. The file
slipped on the material as a skate slips on ice. Evidently we did not
have to do with a glass imitation.
REFRACTION. Knowing now that we had a true hard mineral, it remained to
be determined what mineral it was. On holding the stone in direct
sunlight and reflecting the light onto a white card it was seen at once
that the material was doubly refracting, for a series of _double_ images
of the back facets appeared. These double images might have been
produced by tourmaline as well as by emerald. (Not however by glass
which is singly refracting.) If a direct reading refractometer had been
available the matter could have been settled at once by reading the
refractive indices of the material, for tourmaline and emerald have not
only different refractive indices but have double refraction to
different degrees. Such an instrument was not available at the time and
will hardly be available to most of those who are studying this lesson,
so we can go on with our account of the further testing of the green
stone.
HARDNESS. A test upon the surface of a quartz crystal showed that the
stone was harder than quartz (but so is tourmaline). A true topaz
crystal was too hard for the ring stone, whose edge slipped over the
smooth topaz surface. The green stone was therefore not a green corundum
(Oriental emerald) as the latter has hardness 9 and scratches topaz.
With hardness evidently between 7 and 8 and with double refraction and
with the kind of flaws peculiar to rather brittle minerals we had in all
probability either a tourmaline or an emerald.
DICHROISM. The dichroscope (which might have been used much earlier in
the test but was not at hand at the time) was next tried and the stone
was seen to have marked dichroism--a bluish green and a yellowish green
appearing in the two squares of the instrument when the stone was held
in front of the opening and viewed against a strong light.
As either tourmaline or emerald might thus exhibit dichroism (the
tourmaline more strongly, however, than the emerald) one more test was
tried to finally decide the matter.
SPECIFIC GRAVITY. The stone was removed from its setting and two
specific gravity determinations made by means of a specific gravity
bottle and a fine chemical balance. The two results, which came closely
alike, averaged 2.70 which agrees very nearly with emerald (2.74) and
which is far removed from the specific gravity of tourmaline (3.10). The
stone was now _definitely known_ to be an emerald, as each of several
tests agreed with the properties of emerald, namely:
Color--nearly grass green.
Imperfections--like those of emerald.
Hardness--7-1/2.
Refraction--double.
Dichroism--easily noted.
Specific gravity--2.70.
While one who was accustomed to deal in fine emeralds might not need to
make as detailed an examination of the stone as has just been indicated
above, yet for most of us who do not have many opportunities of studying
valuable emeralds it is safer to make sure by complete tests.
One other concrete example of how to go about testing unknown stones
must suffice to conclude this lesson, after which the student, who has
mastered the separate lessons preceding this, should proceed to test as
many "unknowns" as his time and industry permit in order to really make
_his own_ the matter of these lessons. It may be added here that the
task of testing a stone is much more rapid than this laborious effort to
teach others how to do it might indicate. To one skilled in these
matters only a few seconds are required for the inspection of a stone
with the lens, the dichroscope, or the refractometer, and hardness
tests are swiftly made. A specific gravity test requires more time and
should be resorted to only when there remains a reasonable doubt after
other tests have been applied.
Now for our final example. A red stone, cut in the form of a pear-shaped
brilliant, was submitted to the writer for determination. It had been
acquired by an American gentleman in Japan from an East Indian who was
in financial straits. Along with it, as security for a loan, the
American obtained a number of smaller red stones, a bluish stone, and a
larger red stone. The red stones were all supposed to be rubies. On
examination of the larger red stone with a lens it was at once noted
that the internal structure was that of _scientific ruby_.
TESTING OTHER STONES. Somewhat dashed by the announcement of this
discovery the owner began to fear that all his gems were false.
Examination of the small red stones showed abundance of "silk," a
peculiar fibrous appearance within the stone caused by its internal
structure. The fibers were _straight_ and _parallel_, not _curved_ and
_parallel_ as in synthetic ruby. Tiny bubbles of angular shape also
indicated that the small stones were natural rubies. They exhibited
dichroism and scratched topaz and it was therefore decided that they at
least were genuine.
The pear-shaped brilliant which was first mentioned was of a peculiar,
slightly yellowish, red color. It was very pellucid and free from any
striæ either of the straight or curved types. It had in fact no flaws
except a rather large nick on one of the back surfaces near the girdle.
This was not in evidence from the front of the stone and had evidently
been left by the Oriental gem cutter to avoid loss in weight while
cutting the stone.
The peculiar yellowish character of the red color led us to suspect ruby
spinel. The stone was therefore inspected with the dichroscope and
found to possess no dichroism. The sunlight-card test, too, showed that
the stone was singly refracting.
A test of the hardness showed that the material barely scratched topaz,
but was attacked by sapphire. It was therefore judged to be a red
spinel.
The large bluish stone which the gentleman acquired with the red stones
proved to be iolite, sometimes called cordierite or water-sapphire
(_Saphir d'eau_), a stone seldom seen in this country. It had marked
dichroism--showing a smoky blue color in one direction and a yellowish
white in another. The difference was so marked as to be easily seen
without the dichroscope.
LESSON XVII
SUITABILITY OF STONES FOR VARIOUS TYPES OF JEWELS, AS DETERMINED BY
HARDNESS, BRITTLENESS, AND CLEAVABILITY
HARD STONES NOT NECESSARILY TOUGH. As was suggested in the lesson on
hardness there is prevalent in the public mind an erroneous belief that
hardness carries with it ability to resist blows as well as abrasion.
Now that _it does not follow that because a precious stone is very hard,
it will wear well_, should be made plain. Some rather hard minerals are
seldom or never used as gems, in spite of considerable beauty and
hardness, because of their great brittleness. Other stones, while fairly
hard and reasonably tough in certain directions, have nevertheless so
pronounced a cleavage that they do not wear well if cut, and are
sometimes very difficult to cut at all.
In view of these facts it will be well to consider briefly what stones,
among those most in use, are sufficiently tough as well as hard, to give
good service in jewels, such as rings, which are subject to rough wear.
We may also consider those stones, whose softness, or brittleness, or
ready cleavability, requires that they should be reserved for use only
in those jewels which, because of their nature, receive less rough
usage.
In order to deal with the principal gems systematically, let us consider
them in the order of their hardness, beginning with the hardest gem
material known, which is, of course, diamond.
DURABILITY OF THE DIAMOND. Fortunately this king of gems possesses in
addition to its great hardness, considerable toughness, and although it
is readily cleavable in certain directions it nevertheless requires a
notable amount of force applied in a particular direction to cause it to
cleave. Although sharp knocks will occasionally flake off thin layers
from diamonds when roughly worn in rings, or even in extreme cases
fracture them, yet this happens but seldom and, as the enormous use of
the diamond in ring mountings proves, it is entirely suitable for that
purpose. It follows that, if a stone can stand ring usage, it can safely
be used for any purpose for which precious stones are mounted.
THE CORUNDUM GEMS. Next after the diamond in hardness come the corundum
gems, _i. e._, ruby, sapphire, and the series of corundum gems of colors
other than red and blue. These stones have no noticeable cleavage and
are exceedingly tough, for minerals, as well as very hard. We have only
to consider the use of impure corundum (emery) as a commercial abrasive
in emery wheels, emery cloth, emery paper, etc., to see that the
material is tough. Any of the corundum gems therefore may be used in any
type of jewel without undue risk of wear or breakage. Customers of
jewelers should, however, be cautioned against wearing ruby or sapphire
rings on the same finger with a diamond ring in cases where it would be
possible for the two stones to rub against each other. So much harder
than the ruby is the diamond (in spite of the seeming closeness of
position in Mohs's scale) that the slightest touch upon a ruby surface
with a diamond will produce a pronounced scratch. The possessor of
diamonds and other stones should also be cautioned against keeping them
loose in the same jewel case or other container, as the shaking together
may result in the scratching of the softer materials. The Arabs are said
to have a legend to the effect that the diamond is an _angry_ stone and
that it should not be allowed to associate with other stones lest it
scratch them.
CHRYSOBERYL. Passing on to the next mineral in the scale of hardness we
come to chrysoberyl, which is rated as 8-1/2 on Mohs's scale. This
mineral furnishes us the gem, alexandrite, which is notable for its
power to change in color from green in daylight to red in artificial
light. Chrysoberyl also supplies the finest cat's-eyes (when the
material is of a sufficiently fibrous or tubular structure), and it
further supplies the greenish-yellow stones frequently (though
incorrectly) called "chrysolite" by jewelers. The material is very hard
and reasonably tough and may be used in almost any suitable mounting.
SPINEL. After chrysoberyl come the materials rated as about 8 in
hardness. First and hardest of these is spinel, then comes true or
precious topaz. The various spinels are very hard and tough stones. The
rough material persists in turbulent mountain streams where weaker
minerals are ground to powder, and when cut and polished, spinel will
wear well in any jewel. The author has long worn a ruby spinel in a ring
on the right hand and has done many things that have subjected it to
hard knocks, yet it is still intact, except for a spot that accidentally
came in contact with a fast-flying carborundum wheel, which of course
abraded the spinel.
TOPAZ. The true topaz is a bit softer than spinel, and the rough
crystals show a very perfect basal cleavage. That is, they will cleave
in a plane parallel to the bases of the usual orthorhombic crystals.
This being the case a cut topaz is very likely to be damaged by a blow
or even by being dropped on a hard surface, and it would be wiser not to
set such a stone in a ring unless it was to be but little used, or used
by one who would not engage in rough work while wearing it. Thus a lady
might wear a topaz ring on dress occasions for a long time without
damaging it, but it would not do for a machinist to wear one in a ring.
GEMS BETWEEN 7 AND 8 IN HARDNESS. We now come to a rather long list of
gem minerals ranging between 7 and 8 in hardness. Of these the principal
ones are zircon, almandine garnet, and beryl (emerald and aquamarine)
rated as 7-1/2 in hardness, and pyrope and hessonite garnet rated as
7-1/4 in hardness. Tourmaline and kunzite may also be included in this
group as being on the average slightly above 7 in hardness.
The above minerals are all harder than quartz, and hence not subject to
abrasion by the quartz dust which is everywhere present. In this respect
they are suitable for fairly hard wear. The garnets are of sufficient
toughness so that they may be freely used in rings--and the extensive
use of thin slices of garnet to top doublets proves the suitability of
the material for resisting wear. The zircon is rather more brittle and
the artificially whitened zircons (known as jargoons) are especially
subject to breakage when worn in rings. Fortunately jargoons are not
commonly sold.
The beryl, whether emerald or aquamarine, is rather brittle. Emeralds
are seldom found in river gravels. The material cannot persist in the
mountain streams that bring down other and tougher minerals. The extreme
beauty and value of the emerald has led to its use in the finest
jewels, and the temptation is strong to set it in rings, especially in
rings for ladies. If such rings are worn with the care that valuable
jewels should receive they will probably last a long time without any
more serious damage than the dulling of the sharp edges of the facets
around the table. This slight damage can at any time be repaired by a
light repolishing of the affected facets. If an emerald is already badly
shattered, or as it is called "mossy" in character, it will not be wise
to set it in a ring, as a slight shock might complete its fracture. What
has been said about emerald applies equally to aquamarine except that
the value at stake is much less and the material is usually much freer
from cracks.
Tourmalines, like emeralds, are brittle, and should be treated
accordingly. Here, however, we are dealing with a much less expensive
material than emerald, and if a customer desires a tourmaline in a ring
mounting, while it will be best to suggest care in wearing it, the
loss, in case of breakage, will usually be slight.
Kunzite, like all spodumene, has a pronounced cleavage. It should
therefore be used in brooches, pendants, and such jewels, rather than in
rings. Lapidaries dislike to cut it under some conditions because of its
fragility.
QUARTZ GEMS. Coming down to hardness 7 we have the various quartz gems
and jade (variety jadeite). The principal quartz gems are, of course,
amethyst and citrine quartz (the stone that is almost universally called
topaz in the trade). As crystalline quartz is fairly tough and lacks any
pronounced cleavage, and as it is as hard as anything it is likely to
meet with in use, it is a durable stone in rings or in other mountings.
In the course of time the sharp edges will wear dull from friction with
objects carrying common dust, which is largely composed of powdered
quartz itself, and which therefore gradually dulls a quartz gem. Old
amethysts or "topazes" that have been long in use in rings show this
dulling. There is, however, little danger of fracture with amethyst or
"topaz" unless the blow is severe and then any stone might yield.
The many semi-precious stones which have a quartz basis (such as the
varieties of waxy or cryptocrystalline chalcedony which is largely
quartz in a very minutely crystalline condition) are often even tougher
than the clear crystallized quartz. Carnelian, agate, quartz cat's-eye,
jasper (containing earthy impurities), and those materials in which
quartz has more or less completely replaced other substances, such as
silicified crocidolite, petrified wood, chrysocolla quartz, etc., are
all nearly as hard and quite as tough as quartz itself, and they make
admirable stones for inexpensive rings of the arts and crafts type.
JADE. Jade, of the jadeite variety, which is rarer than the nephrite
jade, and more highly regarded by the Chinese, is an exceedingly tough
material. One can beat a chunk of the rough material with a hammer
without making much impression upon it. It is also fairly hard, about as
hard as quartz, and with the two properties of toughness and hardness it
possesses excellent wearing qualities in any kind of mounting. True
jade, whether jadeite or nephrite, deserves a larger use in inexpensive
ornaments, as it may be had of very fine green color and it is
inexpensive and durable.
SOFTER STONES. Coming next to those minerals whose hardness is 6 or
over, but less than 7, we have to consider jade of the nephrite variety,
demantoid garnet ("olivine" of the trade), peridot (or chrysolite, or
the olivine of the mineralogist), turquoise, moonstone, and opal.
As has already been said of jadeite, the jade of the nephrite variety,
while slightly less hard, is about as tough a mineral as one could
expect to find. It can take care of itself in any situation.
The demantoid garnet (the "olivine" of the trade) is so beautiful and
brilliant a stone that it is a pity that it is so lacking in hardness.
It will do very well for mounting in such jewels as scarf pins,
lavallières, etc., where but little hard wear is met with, but it cannot
be recommended for hard ring use.
The peridot, too, is rather soft for ring use and will last much better
in scarf pins or other mountings little subject to rubbing or to shocks.
Turquoise, although rather soft, is fairly tough, as its waxy luster
might make one suppose, and in addition, being an opaque stone, slight
dulling or scratching hardly lessens its beauty. It may therefore be
used in ring mountings. However, it should be suggested that most
turquoise is sufficiently porous to absorb grease, oil, or other
liquids, and its color is frequently ruined thereby. Of course, such a
change is far more likely to occur to a ring stone than to a turquoise
mounted in some more protected situation.
The moonstone, being a variety of feldspar, has the pronounced cleavage
of that mineral and will not stand blows without exhibiting this
property. Moonstones are therefore better suited to the less rude
service in brooch mountings, etc., than to that of ring stones. However,
being comparatively inexpensive, many moonstones, especially of the
choicer bluish type, are set in ring mountings. The lack of hardness may
be expected to dull their surfaces in time even though no shock starts a
cleavage.
THE OPAL. There remains the opal, of hardness 6, to be considered. As is
well known opal is a solidified jelly of siliceous composition,
containing also combined water. It is not only soft but very brittle and
it will crack very easily. Many opals crack in the paper in which they
are sold, perhaps because of unequal expansion or contraction, due to
heat or cold. In spite of this fragility, thousands of fine opals, and a
host of commoner ones, are set in rings, where many of them
subsequently come to a violent end, and all, sooner or later, become
dulled and require repolishing.
The great beauty of the opal, rivaling any mineral in its color-play,
causes us to chance the risk of damage in order to mount it where its
vivid hues may be advantageously viewed by the wearer as well as by
others.
VERY SOFT STONES. Of stones softer than 6 we have but few and none of
them is really fit for hard service. Lapis lazuli, 5-1/2 in hardness,
has a beautiful blue color, frequently flecked with white or with bits
of fool's gold. Its surface soon becomes dulled by hard wear.
Two more of the softer materials, malachite and azurite, remain to be
described. These are both varieties of copper carbonate with combined
water, the azurite having less water. Both take a good polish, but fail
to retain it in use, being only of hardness 3-1/2 to 4.
LESSON XVIII
MINERAL SPECIES TO WHICH THE VARIOUS GEMS BELONG AND THE CHEMICAL
COMPOSITION THEREOF
Although we have a very large number of different kinds of precious and
semi-precious stones, to judge by the long list of names to be found in
books on gems, yet all these stones can be rather simply classified on
the basis of their chemical composition, into one or another of a
comparatively small number of mineral species. While jewelers seldom
make use of a knowledge of the chemistry of the precious stones in
identifying them, nevertheless such a knowledge is useful, both by way
of information, and because it leads to a better and clearer
understanding of the many similarities among stones whose color might
lead one to regard them as dissimilar.
MINERAL SPECIES. We must first consider what is meant by a "mineral
species" and find out what relation exists between that subject and
chemical composition. Now by a "mineral species" is understood a single
substance, having (except for mechanically admixed impurities)
practically a constant chemical composition, and having practically
identical physical properties in all specimens of it.
DIAMOND AND CORUNDUM. A chemist would call a true mineral a _pure
substance_, just as sugar and salt are pure substances to the chemist.
Thus _diamond_ is a "mineral species," as is also _corundum_. There are
many different colors of both diamond and corundum, but these different
colors are believed to be due to the presence in the pure substance of
impurities in small amounts. Thus every diamond consists mainly of pure
carbon, and all the corundum gems (_ruby_ and the various colors of
_sapphire_) consist mainly of pure oxide of aluminum. The properties of
all diamonds are practically alike and so are the properties of all the
corundum gems whether red (ruby), blue (sapphire), yellow (Oriental
topaz), green (Oriental emerald), or purple (Oriental amethyst).
Thus all diamonds, of whatever color, belong to the one species,
diamond, and in this case the usual custom in naming them agrees with
the facts. Similarly all sapphires, of whatever color, belong to the
mineral species "corundum." Thus a ruby is a red corundum.
The old French traveler and gem merchant, Tavernier, tells us that in
the seventeenth century, when he visited the mines of Pegu, the natives
knew of the similarity of the corundum gems and even called all by one
name, with other names attached to designate the color. Singularly
enough, the common name used by them was _ruby_ rather than sapphire, as
now. Thus they called blue corundum gems blue rubies; yellow corundums,
yellow rubies, etc.
It is easily seen that if one recognizes the similar nature of all the
many colors and shades of corundum that the number of things that one
has to remember in order to be well acquainted with these stones is
considerably diminished. Thus, instead of having a whole series of
specific gravities to remember one has only to remember that all the
corundum gems have a specific gravity of approximately 4. Similarly they
are all of practically the same refractive index (1.761-1.770, being
doubly refracting) that they all exhibit dichroism when at all deeply
colored, etc.
Having thus indicated what we mean by mineral species and having
illustrated the matter by the cases of diamond and corundum and further
having stated that all diamonds are composed of pure carbon (except for
traces of impurities) and all corundum gems mainly of oxide of aluminum,
we may proceed to consider other mineral species and find out what gems
they afford us.
CARBON, THE ONLY ELEMENT FURNISHING A GEM. It will be noted that the
first species considered, diamond, consisted of but a single element,
carbon. It is thus exceedingly simple in composition, being not only a
pure substance but, in addition, an elementary substance. CORUNDUM, the
second species considered, was a little more complex, having two
elements, aluminum and oxygen, in its make-up, but completely and
definitely combined in a new compound that resembles neither aluminum
nor oxygen. It is thus a compound substance. No other element than
carbon affords any gem-stone when by itself.
OXIDES OF METALS. There is, however, another oxide, in addition to
aluminum oxide, that furnishes gem material. It is _silicon oxide_,
containing the two elements silicon and oxygen. Silicon itself is a
dark, gray, crystalline element that seems half metallic, half
non-metallic in its properties. It is never found by itself in nature
but about twenty-eight per cent. of the crust of the earth is composed
of it in compound forms, and one of the most abundant of these is
QUARTZ, which is a mineral species, and which contains just silicon and
oxygen. That is, it is oxide of silicon. Now quartz is colorless when
pure (_rock crystal_), but it is frequently found colored purple
(probably by oxide of manganese) and it is then called _amethyst_ by the
jeweler. At other times its color is yellow (due to oxide of iron) and
then the jeweler is prone to call it "_topaz_," although properly
speaking that name should, as we shall soon see, be reserved for an
entirely different mineral species. _Chalcedony_ too (which when banded
furnishes us our _agates_, and when reddish our _carnelian_) is a
variety of quartz, and _prase_ is only quartz colored green by fibers of
actinolite within it.
The common _cat's-eye_ and the _tiger's-eye_ are varieties of quartz
enclosing fibrous minerals or replacing them while still keeping the
arrangement that they had. "_Venus hair stone_" is quartz containing
needle-like crystals of rutile, and "_iris_" is quartz that has been
crackled within, so as to produce rainbow colors, because of the
effects of thin layers of material. _Aventurine quartz_ (sometimes
called goldstone) has spangles of mica or of some other mineral enclosed
in it. The _jaspers_ are mainly quartz with more of earthy impurity than
the preceding stones.
Thus all this long list of stones of differing names can be classified
under the one mineral species, quartz. Together they constitute the
quartz gems. In properties they are essentially alike, having specific
gravity 2.66, hardness 7, slight double refraction, etc., the slight
differences that exist being due only to the presence of varying amounts
of foreign matter.
OPAL. The _opal_ may be considered along with the quartz gems, because,
like them, it is composed mainly of oxide of silicon, but the opal also
has water combined with the silicon oxide (not merely imprisoned in it).
Thus opal is a hydrous form of silica (hydrous comes from the Greek word
for water).
SPINEL. All our other stones are of more complicated chemical
composition than the preceding. Coming now to mineral species which have
three chemical elements in them we may consider first _spinel_, which
has the two metallic elements aluminum and magnesium and the
non-metallic element oxygen in it. It is virtually a compound of the two
oxides, aluminum oxide and magnesium oxide. The variously colored
spinels, like the various corundums, all have the same properties, thus
they are all of hardness 8 or a little higher, they all have single
refraction, and all have specific gravity 3.60.
CHRYSOBERYL. Another mineral species which, like spinel, has just three
elements in its composition is _chrysoberyl_. This mineral contains the
metals aluminum and beryllium combined with the non-metal oxygen. Thus
it is really to be regarded as a compound of the two oxides, aluminum
oxide and beryllium oxide. This species furnishes us _Alexandrite_,
_chrysoberyl cat's-eye_ and less valuable chrysoberyls of
yellowish-green color. All are of the one species, the marked color
difference being due to the presence of different impurities. The
cat's-eye effect in one of the varieties is due to the internal
structure rather than to the nature of the material.
THE SILICATES. Nearly all of the remaining precious stones belong to a
great group of mineral species known as the silicates. These are so
called because they consist largely of oxide of silicon (the material
above referred to under quartz gems). This oxide of silicon is not free
and separate in the silicates but is combined chemically with other
oxides, chiefly with metallic oxides. Thus there are many different
silicates because, in the earth, many different metallic oxides have
combined with silicon oxide. Also in many cases two or three or even
more metallic oxides have combined with silicon oxide to make single new
compounds.
GLASS, A MIXTURE OF SILICATES. Those who are familiar with glass making
may receive some help at this point by remembering that the various
glasses are silicates, for they are made by melting sand (which is
nearly pure oxide of silicon) with various metallic oxides. With lime
(calcium oxide) and soda (which yields sodium oxide) we get soda-lime
glass (common window glass). Lead oxide being added to the mixture a
dense, very brilliant, but soft glass (flint glass) results. Cut glass
dishes and "paste" gems are made of this flint glass. Now the glasses,
although they are silicates, are not crystalline, but rather they are
_amorphous_, that is, without any definite structure. Nature's
silicates, on the other hand, are usually crystallized or at least
crystalline in structure. (In a few cases we find true glasses, volcanic
glass, or obsidian, for example.)
Having thus introduced the silicates we may now consider which ones
among the many mineral silicates furnish us with precious or
semi-precious stones.
BERYL, EMERALD, AND AQUAMARINE. First in value among the silicates is
_beryl_, which, when grass green, we call _emerald_. The _aquamarine_
and _golden beryl_ too belong to this same species. Beryl is a silicate
of aluminum and beryllium. That is, it is a compound in which oxide of
silicon is united with the oxides of aluminum and of beryllium. There
are thus four chemical elements combined in the one substance and it is
hence more complicated in its composition than any of the gems that we
have yet considered. It is worthy of note that aluminum occurs in the
majority of precious stones, the only species so far considered that
lack it being diamond, and the quartz gems.
Perhaps the silicates that are next in importance to the jeweler, after
beryl, are those which form the _garnets_ of various types. There are
four principal varieties of garnet (although specimens of garnet
frequently show a crossing or blending of the types).
GARNETS. The types are (1) _Almandite_ garnet; (2) _Pyrope_ garnet; (3)
_Hessonite_ garnet; and (4) _Andradite_ garnet. These are all silicates,
the almandite garnets being silicates of iron and aluminum; the pyrope
garnets are silicates of magnesium and aluminum; the hessonite garnets,
silicates of calcium and aluminum, and the andradite garnets, silicates
of calcium and iron.
The so-called almandine garnets of the jeweler are frequently of the
almandite class and tend to purplish red. The pyrope garnets are, as the
name literally implies, of fire red color, as a rule, but they also may
be purplish in color. The hessonite garnets are frequently brownish red
and are sometimes called "cinnamon stones." The andradite garnets
furnish the brilliant, nearly emerald green demantoids (so often called
"_olivine_" by the trade).
Thus all the garnets are silicates and yet we have these four principal
mineral species, which, however, are more closely related to each other
in crystal form, in character of composition and in general properties,
than is usual among the other silicates. Specimens which have any one of
the four types of composition unblended with any of the other types
would be found to be exactly alike in properties. As was suggested
above, however, there is a great tendency to blend and this is well
illustrated by the magnificent _rhodolite_ garnets, of rhododendron hue
which were found in Macon County, North Carolina. These had a
composition between almandite and pyrope, that is, they had both
magnesium and iron with aluminum and silica.
The true TOPAZ next calls for consideration as it too is a silicate. The
metallic part consists of aluminum, and there are present also the
non-metals fluorine and hydrogen. Here we have five elements in the one
substance. Various specimens of this species may be wine yellow, light
blue, or bluish green, pink or colorless, yet they all have essentially
the same properties.
TOURMALINE is about as complicated a mineral as we have. It is a very
complex silicate, containing aluminum, magnesium, sodium (or other
alkali metal, as, for example, lithium), iron, boron, and hydrogen. As
Ruskin says of it in his _The Ethics of the Dust_, when Mary asks "and
what is it made of?" "A little of everything; there's always flint
(silica) and clay (alumina) and magnesia in it and the black is iron,
according to its fancy; and there's boracic acid, if you know what that
is: and if you don't, I cannot tell you to-day and it doesn't signify;
and there's potash and soda; and on the whole, the chemistry of it is
more like a mediæval doctor's prescription, than the making of a
respectable mineral." The various tourmalines very closely resemble each
other in their properties, the slight differences corresponding to
differences in composition do not alter the general nature of the
material.
MOONSTONE belongs to a species of mineral known as feldspar. The
particular feldspar that furnishes most of the moonstone is orthoclase,
a silicate of potassium and aluminum. Another feldspar sometimes seen as
a semi-precious stone is _Labradorite_. _Amazonite_, also, is a
feldspar. _Sunstone_ is a feldspar which includes tiny flakes or
spangles of some other mineral.
The mineral species _olivine_ gives us _peridot_. It is a silicate of
magnesium.
ZIRCON is itself a species of mineral and is a silicate of zirconium.
The names _hyacinth_, _jacinth_, and _jargoon_ are applied to red,
yellow, and colorless zircon in the order as given.
JADE may be of any of several different species of minerals, all of
which are very tough. The principal jades belong, however, to one or the
other of two species, _jadeite_ and _nephrite_. Jadeite is a sodium
aluminum silicate and nephrite, a calcium magnesium silicate.
Leaving the silicates we find very few gem minerals remaining. The
phosphates furnish us _turquoise_, a hydrous aluminum phosphate, with
copper and iron. _Variscite_ is also a phosphate (a hydrated aluminum
phosphate).
The carbonates give us _malachite_ and _azurite_, both carbonates of
copper with combined water, the malachite having more water.
LESSON XIX
THE NAMING OF PRECIOUS STONES
Owing to the confusion which may result from a lack of uniformity in the
naming of precious stones, it is very desirable that jewelers and stone
merchants inform themselves in regard to the correct use of the names of
the gems, and that they use care in speaking and in writing such names.
As nearly all precious and semi-precious stones are derived from a
relatively small number of _mineral species_, as we saw in Lesson
XVIII., and as the science of _mineralogy_ has a very orderly and
systematic method of naming the minerals, the best results are had in
the naming of gems when we use, as far as is possible, the language of
mineralogy.
ANCIENT USAGE. Long established custom and usage, however, must be
observed, for any system of naming must be generally understood in order
to be useful. Thus the proper name for blood red, crystallized oxide of
aluminum, of gem quality, according to the mineralogical system of
naming, would be red _corundum_, but that same material is referred to
in the Old Testament thus (in speaking of wisdom), "She is more precious
than _rubies_." It is obviously necessary to keep and to use all such
terms as have been for years established in usage, even though they do
not agree with the scientific method of naming the particular mineral.
It is, however, necessary that any name, thus retained, should be
correctly used, and that it should not be applied to more than one
material. Thus the term _ruby_ should be reserved exclusively for red
corundum, and not applied to other red minerals such as garnet, spinel,
etc., as is too often done.
It will be the purpose of this lesson to attempt to set forth as clearly
and as briefly as possible what constitutes good usage in the naming of
the principal stones, and also to point out what incorrect usage is most
in need of being avoided.
To cover the subject systematically we will adopt the order of hardness
that we did in discussing mineral species in Lesson XVIII.
FANCY DIAMONDS. Beginning with the hardest of all gems, the _diamond_,
we have no difficulty as regards naming, as all specimens of this
mineral, regardless of color, are called diamonds. When it is necessary
to designate particular colors or tints, or differences in tint,
additional names are used--for example, all diamonds of pronounced and
pleasing color are called "fancy" diamonds in the trade. Certain of
these "fancy" diamonds are still further defined by using a name
specifying the color, as, for example, "canary" diamonds (when of a fine
bright yellow), or "golden fancies," when of a fine golden brown, or
"orange," or "pink," or "absinthe green," or "violet," as the case may
be.
NAMES OF VARIOUS GRADES OF WHITE DIAMONDS. The great majority of the
diamonds which come on the market as cut stones belong, however, to the
group which would be spoken of as white diamonds, but many qualifying
names are needed to express the degree of approach to pure white
possessed by different grades of these diamonds. Thus the terms: 1,
_Jägers_; 2, _Rivers_; 3, _Blue Wesseltons_; 4, _Wesseltons_; 5, _Top
Crystals_; 6, _Crystals_; 7, _very light brown_; 8, _Top Silver Capes_;
9, _Silver Capes_; 10, _Capes_; 11, _Yellows_, and 12, _Browns_,
describe _increasing_ depth of color, and hence _decreasing_ value in
diamonds.
POPULAR NAMES. Certain more popular names for diamonds of differing
degrees of whiteness may next be set forth. The term "blue white" (a
much abused expression, by the way) should be applied only to diamonds
of such a close approach to pure whiteness of body substance, as seen
on edge in the paper that, when faced up and undimmed, they give such a
strong play of _prismatic_ blue that any slight trace of yellow in their
substance is completely disguised, and the effect upon the eye is
notably blue. This would be the case with stones of the grades from 1
through 4 in the list above. Grades 5 and 6 might properly be called
"_fine white_," and grades 7, 8, and 9 simply "_white_." Grade 10 is
frequently spoken of as "_commercial white_," and grade 11 sometimes as
"off color." Grade 12 includes all degrees of brownness except the very
light shades and the deep, pretty shades of the "fancy" browns.
RUBIES. Leaving the naming of the different colors of diamonds we come
to the gems furnished us by the mineral known as _corundum_. As we have
previously seen, this mineral occurs in many different colors and with
wide differences of tint and shade in each of the principal colors. The
best practice with regard to naming the corundum gems is to call the
red material, when of a good, full red of pleasing shade, _ruby_. The
finest shades of blood red are usually called "_Burmah rubies_" because
more rubies of this quality are found in Burmah than anywhere else. Any
ruby of the required shade would, however, be called a Burmah ruby in
the trade regardless of its geographical origin. The most desirable tint
among Burmah rubies is that which is known as "pigeon blood" in color.
This color is perhaps more accurately defined as like the color in the
center of the red of the solar spectrum. Certain slightly deeper red
rubies are said to be of "beef blood" color. The English are said to
prefer these. Those of slightly lighter tint than pigeon blood are
sometimes referred to as of "French color," from the fact that they are
preferred by French connoisseurs.
Rubies of dark, garnet-like shade are known as "_Siam rubies_," many
such being found in that country. Light pinkish rubies are called
"_Ceylon rubies_." It should be clearly kept in mind that all these
"rubies" are of red corundum, and that in all their distinctive
properties except color they are essentially similar.
SAPPHIRES. Corundum of fine blue color is known as "_sapphire_." The
"cornflower blue" seems to be most in favor at present. Such sapphires
are sometimes called "_Kashmir sapphires_" because many fine ones come
from that State. "_Ceylon sapphires_" are usually paler than the
cornflower blue. "_Montana sapphires_" are usually of greenish blue or
pale electric blue. Such fine blue stones as are mined in Montana would
be sold under another name according to the quality of their color, and
not as "Montana sapphires." "_Australian sapphires_" are of a very deep,
inky blue, and do not command a high price. Here again, as with rubies,
the classification depends upon the color rather than upon the origin,
although the geographical names that are used, correctly state the
usual source of stones of the particular color.
All corundums other than ruby and blue sapphire are usually called by
the term "sapphire," with a qualifying adjective designating the color;
thus we may have pink sapphire, golden sapphire, green sapphire, etc.
When of very fine yellow color the yellow sapphire is sometimes called
"_Oriental topaz_" by jewelers, the term "_Oriental_" as thus used
indicating that the material is corundum. We also have "_Oriental
amethyst_" and "_Oriental emerald_" for the purple, and the fine green,
and "_Oriental aquamarine_" for the light blue-green corundum. The
yellow corundum is also sometimes called "_King topaz_," especially in
Ceylon. Inferior sapphires of almost every conceivable color are
frequently assorted in lots and sold as "fancy sapphires." Such lots,
however, almost always need reclassification as they often contain as
many as a dozen mineral species besides corundum.
Sapphires and rubies of minute tubular internal structure frequently
display a beautiful six-pointed star when cut to a round-topped cabochon
shape and exposed to direct sunlight or to light from any other single
source. Such stones are named "_star sapphire_" and "_star ruby_."
The artificial rubies and sapphires should all be called _scientific
ruby_ or _sapphire_, and not "_reconstructed_" or "_synthetic_" as none
are made to-day from small, real rubies, and as the process is in no
sense a chemical synthesis.
CHRYSOBERYL. Leaving the corundum gems we come next to chrysoberyl. When
the gems furnished by this mineral are of a fine green by daylight, and
of a raspberry red by artificial light, as is sometimes the case, they
should be called "_Alexandrites_" (after the Czar Alexander II., in
whose dominions, and on whose birthday, the first specimens are said to
have been discovered). When chrysoberyl is of fibrous or tubular
internal structure it affords cat's-eyes (when cabochon cut), and these
should be specifically named as "_chrysoberyl cat's-eye_" to distinguish
them from the less beautiful and less valuable quartz cat's-eyes. Other
varieties of chrysoberyl (most of those marketed are of a
greenish-yellow color) are correctly named simply "_chrysoberyls_." Such
stones are, however, sometimes incorrectly called "_chrysolite_" by the
trade, and this practice should be corrected, as the term chrysolite
applies correctly only to the mineral _olivine_ which gives us the
_peridot_.
SPINEL. Next in the order that we have chosen comes "_spinel_." The more
valuable spinels are of a red color that somewhat closely approaches the
red of some rubies. Such red spinels should be called "_Ruby spinel_"
(and _not spinel ruby_). The stones themselves sometimes get mixed with
corundum rubies (they are frequently found in the same gem gravels), and
this makes it all the more necessary that both stones and names should
be clearly distinguished. Some dealers call reddish spinels "_Balas
ruby_" (rose red), and orange red ones "_rubicelle_." Violet red spinel
is sometimes called "_almandine spinel_." It is very desirable that the
name of the mineral species, _spinel_, should be used, together with a
qualifying color adjective, in naming gems of this species, rather than
such terms as "rubicelle," "balas ruby," "spinel ruby," etc.
TOPAZ. We come now to _topaz_. True, or _precious topaz_, as it is
usually called, to distinguish it from the softer and less valuable
yellow quartz, is seldom seen in the trade to-day. Jewelers almost
always mean yellow quartz when they speak of "topaz." This is an
unfortunate confusion of terms, and one which will be hard to eradicate.
There is seldom any injustice done through this misnaming, as the price
charged is usually a fair one for the material offered. Considerably
higher prices would be necessary if true topaz was in question.
An instance from the writer's experience will serve to illustrate the
confusion that exists in the trade as to what should be called topaz. A
jeweler of more than ordinary acquaintance with gems exhibited some fine
brooch stones as specimens of topaz. On remarking that they were of
course _citrine quartz_ rather than _true topaz_, the author was met
with the statement that the brooch stones were _real_ topaz. In order to
make clear to the dealer the difference between the two species, the
author asked him if he hadn't some smaller topazes in stock that had
cost him considerably more than the brooch stones. The dealer replied
that he had some small wine yellow topazes for which he had paid more,
and he produced them. The latter stones were true Brazilian topazes.
Most of them had tiny, crackly flaws in them, as is frequently the case,
and, as the writer pointed out to the dealer, they had been bought by
the _carat_, whereas the large brooch stones had been bought at a
certain price per _pennyweight_. In fact the little stones had cost more
per carat than the larger ones had per pennyweight.
The dealer was then asked if there must not be some difference in the
real nature of the two lots to justify paying more per carat for small,
imperfect stones than per pennyweight for large perfect ones. He of
course acknowledged that it would appear reasonable that such was the
case. He was next shown that his small _true topazes_ scratched his
large stones easily, but the large ones could get no hold upon the
surfaces of the small ones. (It will be remembered that topaz has a
hardness of 8, while quartz has a hardness of 7.) The explanation then
followed that the two lots were from two entirely distinct minerals,
topaz and quartz, and that the former was harder, took a somewhat better
polish, and was more rare (in fine colors) than quartz. Of course the
yellow quartz should be sold under the proper name, _citrine quartz_.
(From the same root that we have in "_citrus_" as applied to fruits.
For example the "California Citrus Fruit Growers' Association," which
sells oranges, lemons, grape fruit, etc. The color implication is
obvious.) If the jeweler still wishes to use the term "topaz" because of
the familiarity of the public with that name, then he should at least
qualify it in some way. One name that is current for that purpose is
"Spanish topaz," another is "Quartz-topaz." Perhaps the latter is the
least objectionable of the names that include the word topaz.
Some of the wine yellow true topazes lose the yellow, but retain the
pink component, on being gently heated. The resulting pink stone is
rather pretty and usually commands a higher price than the yellow
topazes. Such artificially altered topazes should be sold only for what
they are, and probably the name "pinked topaz," implying, as it does,
that something has been done to the stone, is as good a name as any.
There is, however, little chance of fraud in this connection, as
natural pink topazes are not seen in the trade, being very rare.
Some bluish-green topaz is said to be sold as aquamarine, and this
confusion of species and of names should, of course, be stopped by an
actual determination of the material as to its properties. Lacking a
refractometer, the widely differing specific gravities of the two
minerals would easily serve to distinguish them.
LESSON XX
THE NAMING OF PRECIOUS STONES (_Concluded_)
BERYL, EMERALD, AQUAMARINE. Coming now to _beryl_ we have first
_emerald_, then _aquamarine_, then beryls of other colors to consider.
There is too often a tendency among dealers to confuse various green
stones, and even doublets, under the name _emerald_. While the price
charged usually bears a fair relation to the value of the material
furnished, it would be better to offer tourmaline, or peridot (the
mineral name of which is olivine), or demantoid garnet (sometimes
wrongly called "Olivine"), or "emerald doublets," or emerald or
"imitation emerald," as the case might be, under their own names.
There are no true "synthetic" or "scientific" or "reconstructed"
emeralds, and none of these terms should be used by the trade. There
has been an effort made in some cases to do business upon the good
reputation of the scientific rubies and sapphires, but the products
offered, when not out and out glass imitations, have usually been
doublets or triplets, consisting partly of some pale, inexpensive,
natural mineral, such as quartz or beryl, and a layer of deep green
glass to give the whole a proper color. All attempts to melt real
emerald or beryl have yielded only a _beryl glass_, softer and lighter
than true emerald, and not _crystalline_, but rather glassy in
structure. Hence the names "reconstructed," "synthetic" and "scientific"
should never be applied to emerald.
The light green and blue green beryls are correctly called
_aquamarines_, the pale sky-blue beryls should be named simply _blue
beryl_. Yellow beryl may be called _golden beryl_, or it may be called
"_heliodor_," a name that was devised for the fine yellow beryl of
Madagascar. Beautiful pink beryl from Madagascar has been called
"_morganite_," a name that deserves to live in order to commemorate the
great interest taken by J. Pierpont Morgan in collecting and conserving
for future generations many of the gems in the American Museum of
Natural History in New York.
ZIRCON. We now come to a number of minerals slightly less hard than
beryl, but harder than quartz, and _zircon_ is perhaps as hard as any of
these, so it will be considered next. Red zircon, which is rare, is
properly called "_hyacinth_." Many Hessonite garnets (cinnamon stones)
are incorrectly called hyacinths, however. The true hyacinth has more
snap and fire owing to its adamantine surface luster and high dispersive
power, as well as to its high refractive index. A true hyacinth is a
beautiful stone. Golden yellow zircons are correctly called
"_jacinths_." Artificially whitened zircons (the color of which has been
removed by heating) are known as "jargoons" or sometimes as "Matura
diamonds." All other colors in zircon should be named simply zircon,
with a color adjective to indicate the particular color as, "brown
zircon," etc.
TOURMALINE. Tourmaline furnishes gems of many different colors. These
are all usually called simply tourmaline, with a color adjective to
specify the particular color, as, for example, the "pink tourmaline" of
California. Red tourmaline is, however, sometimes called "_rubellite_,"
and white tourmaline has been called "_achroite_." The latter material
is seldom cut, and hence the name is seldom seen or used.
GARNET. We may next consider the _garnets_, as most of them are somewhat
harder than quartz. As was said in Lesson XVIII. in our study of mineral
species, there are several types of garnets, characterized by similarity
of chemical composition, or at least by analogy of composition, but,
having specific differences of property. The names used by jewelers for
the several types of garnets ought to be a fairly true indication as to
the type in hand in a particular case. At present there is considerable
confusion in the naming of garnets. The most common practice is to call
all garnets of a purplish-red color "almandines." As many such garnets
belong to the mineral species _almandite garnet_, there is little
objection to the continuance of this practice. The somewhat less dense,
and less hard blood red garnets are properly called "_pyrope garnets_"
(literally "fire" garnets). Many of the Arizona garnets belong in this
division. The term "Arizona _rubies_" should _not_ be used. As was said
under ruby, nothing but red corundum should receive that title.
Similarly the pyrope garnet of the diamond mines of South Africa is
incorrectly called "Cape ruby." Pyrope and almandite garnet tend to
merge in composition and in properties, and the beautiful "_Rhodolite_"
garnets of Macon County, North Carolina, are between the two varieties
in composition, in color, and in other properties.
_Hessonite garnet_ furnishes yellowish-red and brownish-red stones,
which are sometimes also called "cinnamon stones." They are also
frequently and incorrectly called jacinth or hyacinth, terms which, as
we have seen, should be reserved for yellow and red zircon,
respectively.
_Andradite garnet_ furnishes brilliant green stones, which have been
incorrectly named "Olivines" by the trade. The name is unfortunate as it
is identical with the true name of the mineral which gives us peridot.
The name does not even suggest the color of these garnets correctly, as
they are seldom olive green in shade. As the scarcity of fine specimens
and their great beauty make a fairly high price necessary, the public
would hardly pay it for anything that was called "garnet," as garnets
are regarded as common and cheap. Perhaps the adoption of the name
"_Demantoid_" might relieve the situation. The stones are frequently
referred to as "demantoid garnets" on account of their diamond-like
luster and dispersion. The use of "demantoid" alone, if a noun may be
made from the adjective, would avoid both the confusion with the mineral
olivine, and the cheapening effect of the word garnet, and would at the
same time suggest some of the most striking properties of the material.
"_Spodumene_" furnishes pink to lilac "_Kunzite_," named after Dr.
George F. Kunz, the gem expert, and for a time an emerald green variety
was had from North Carolina which became known as "_Hiddenite_," after
its discoverer, W. E. Hidden. No confusion of naming seems to have
arisen in regard to this mineral.
The next mineral in the scale of hardness is quartz. (Hardness 7.) When
pure and colorless it should be called "_rock crystal_." Purple quartz
is of course _amethyst_. Some dealers have adopted a bad practice of
calling the fine deep purple amethyst "Oriental" amethyst, which should
not be done, as the term "Oriental" has for a long time signified a
_corundum_ gem. As Siberia has produced some very fine amethysts, the
term "_Siberian amethyst_" would be a good one to designate any
especially fine gem.
QUARTZ GEMS. We have already considered the naming of yellow quartz in
connection with topaz. "_Citrine quartz_" is probably the best name for
this material. If it is felt that the name "topaz" must be used, the
prefix "quartz" should be used, or perhaps "Spanish topaz" will do, but
some effort should be made to distinguish it from the true precious
topaz. In addition to amethyst and citrine quartz we have the pinkish,
milky quartz known as "_rose quartz_." This is usually correctly named.
"_Cat's-eye_" is a term that should be reserved for the Chrysoberyl
variety, and the quartz variety should always be called "quartz
cat's-eye." "_Tiger's-eye_" is a mineral in which a soft fibrous
material has been dissolved away, and quartz has been deposited in its
place. "_Aventurine quartz_" is the correct name for quartz containing
spangles of mica. Clear, colorless pebbles of quartz are sometimes cut
for tourists. Such pebbles are frequently misnamed "diamonds" with some
prefix, as for example "Lake George diamonds," etc. Among the minutely
crystalline varieties of quartz we have the clear red, which should be
called "_carnelian_," the brownish-red "_sard_," the green
"_chrysoprase_," the leek green "_prase_," and the brighter green
"_plasma_." The last three are not so commonly seen as the first two,
and frequently the best-colored specimens are artificially dyed.
"_Jasper_," a material more highly regarded by the ancients than at
present, is mainly quartz, but contains enough earthy material to make
it opaque. "_Bloodstone_" is a greenish chalcedony with spots of red
jasper.
"_Agates_" are banded chalcedonies, the variety called "_onyx_" having
very regular bands, and the "_sardonyx_" being an onyx agate in which
some of the bands are of reddish sard.
Just as we considered opal with quartz (because of its chemical
similarity) when discussing mineral species, so we may now consider the
proper naming of opals here. "_Precious opal_" is distinguished from
"_common opal_" by the beauty of its display rather than by any
difference in composition. The effect is of course due to the existence
of thin films (probably of material of slightly different density),
filling what once were cracks in the mass. The rainbow colors are the
result of interference of light (see a college text on physics for an
explanation of interference). The varying thickness of these films gives
varying colors, so different specimens of opal show very different
effects. The differences of distribution of the films within the
material also cause variations in the effects. Hence we have hardly any
two specimens of opal that are alike.
There are, however, certain fairly definite types of opal and jewelers
should learn to apply correct names to these types. Most prominent
among the opals of to-day are the so-called "_Black opals_" from New
South Wales. These give vivid flashes of color out of seeming darkness.
In some positions the stones, as the name implies, appear blue-black or
blackish gray. By transmitted light, however, the bluish stones appear
yellow. Owing to the sharp contrast between the dark background and the
flashing spectrum colors, black opals are most attractive stones and
fine specimens command high prices. One fine piece, which was on
exhibition at the Panama-Pacific Exposition was in the shape of an
elongated shield, about 1-3/4 inches by 1-1/8 inches in size and rather
flat and thin for its spread. It gave in one position a solid surface of
almost pure ruby red which changed to green on tipping the stone to the
opposite direction; $2,000 was asked for the piece.
"_White opal_" is the name applied to the lighter shades of opal which
do not show the bluish-black effect in any position. "_Harlequin opal_"
has rather large areas of definite colors giving somewhat the effect of
a map of the United States in which the different States are in
different colors.
"_Fire opal_" is an orange-red variety. It has some "play" of colors in
addition to its orange-red body color.
"_Opal Matrix_" has tiny specks and films of precious opal distributed
through a dark volcanic rock and the mass is shaped and polished as a
whole.
JADE. "_Jade_" should next receive attention. It is a much abused term.
Under it one may purchase _jadeite_, _nephrite_, _bowenite_,
_amazonite_, or frequently simply _green glass_. The use of the word
ought to be confined to the first two minerals mentioned, namely,
jadeite and nephrite, for they only possess the extreme toughness
together with considerable hardness that we expect of jade. Bowenite,
while tough, is relatively soft and amazonite is brittle and also easily
cleavable, while glass is both soft and brittle.
PERIDOT AND OLIVINE. The mineral "_olivine_" gives us the "_peridot_"
(this name should be kept for the deeper bottle green stones), and the
olive green gems of this same mineral may correctly be called
"_olivine_" or "_chrysolite_." As was explained under garnet, jewelers
frequently use the term "olivine" to designate demantoid garnet. The
term chrysolite is also sometimes incorrectly used for the
greenish-yellow chrysoberyl.
FELDSPAR GEMS. Among the minerals softer than quartz, which are used as
gems, we have also "_feldspar_," which gives us "_moonstone_,"
"_Labradorite_," and "_Amazonite_."
An opalescent form of chalcedony is frequently gathered on California
beaches and polished for tourists under the name of "_California
Moonstone_." This name is unfortunately chosen as the material is not
the same as that of true moonstone and the effect is not so pronounced
or so beautiful. The polished stones show merely a milky cloudiness
without any of that beautiful sheen of the true moonstone.
"_Labradorite_" is usually correctly named. "_Amazonite_" was originally
misnamed, as none is found along the river of that name. The term has
come into such general use, however, that we shall probably have to
continue to use it, especially as no other name has come into use for
this bluish-green feldspar. As has already been said, amazonite is
sometimes sold as "jade," which is incorrect.
MALACHITE, AZURITE, AND LAPIS LAZULI. _Malachite_ and _azurite_ are
usually correctly named, but "_lapis lazuli_" is a name that is
frequently misused, being applied to crackled quartz that has been
stained with Prussian blue, or some other dye, to an unconvincing
resemblance to true lapis. Such artificially produced stones are
sometimes sold as "_Swiss lapis_." They are harder than true lapis and
probably wear much better in exposed ornaments, but they are not lapis
and are never of equal color, and names should not be misused, and
especially is this true in a trade where the public has had to rely so
completely upon the knowledge and the integrity of the dealer.
With the increase of knowledge about precious stones that is slowly but
steadily growing among the public, it becomes more than ever necessary
for the jeweler and gem dealer to know and to use the correct names for
all precious stones. The student who wishes to learn more about the
matter will have to cull his information from many different works on
gems. G. F. Herbert-Smith, in his _Gem-Stones_, gives a three and one
half page chapter on "Nomenclature of Precious Stones" (Chap. XIII., pp.
109-112). The present lesson has attempted to bring together in one
place material from many sources, together with some suggestions from
the author.
LESSON XXI
WHERE PRECIOUS STONES ARE FOUND
OCCURRENCE OF DIAMOND. Every dealer in precious stones should know
something of the sources of the gems that he sells. The manner of the
occurrence of the rough material is also a matter of interest. It will
therefore be the purpose of this lesson to give a brief account of the
geographical sources of the principal gems and of their mode of
occurrence in the earth.
For the sake of uniformity of treatment we will once more follow the
descending order of hardness among the gems and we thus begin by
describing the occurrence of diamond. It will be of interest to note
first that the earliest source of the diamond was India, and that for
many years India was almost the sole source. Tavernier tells us that the
diamond mining industry was in a thriving state during the years from
1640 to 1680, during which time he made six journeys to India to
purchase gems. He speaks of Borneo as another source of diamonds, but
most of the diamonds of that time were furnished by India.
"GOLCONDAS." Indian diamonds were noteworthy for their magnificent
steely blue-white quality and their great hardness, and occasionally one
comes on the market to-day with an authentic pedigree, tracing its
origin back to the old Indian mines, and such stones usually command
very high prices. One of a little over seven and one half carats in
weight, in the form of a perfect drop brilliant, has lately been offered
for sale at a price not far from $1,000 per carat. Such diamonds are
sometimes called "Golcondas" because one of the mining districts from
which the fine large Indian stones came was near the place of that name.
Some of the stones from the Jägersfontein mine in South Africa resemble
the Golcondas in quality. Many of the large historical crown diamonds
of Europe came from the Indian mines.
The stones were found in a sedimentary material, a sort of conglomerate,
in which they, together with many other crystalline materials, had
become imprisoned. Their original source has never been determined. They
are therefore of the so-called "River" type of stone, having probably
been transported from their original matrix, after the disintegration of
the latter, to new places of deposit, by the carrying power of river
waters.
The Indian mines now yield very few stones. The United States Consular
reports occasionally mention the finding of a few scattered crystals but
the rich deposits were apparently worked out during the seventeenth
century and the early part of the eighteenth century.
In 1725 and in the few following years the Brazilian diamond fields
began to supersede those of India. Like the latter, the Brazilian fields
were alluvial, that is, the materials were deposited by river action
after having been carried to some distance from their original sources.
BRAZILIAN DIAMONDS. The diamonds of Brazil also resembled those of India
in quality, being on the average better than those of the present South
African mines. It may be added that even the African diamonds that are
found in "river diggings" average better in quality than those of the
volcanic pipes which form the principal source of the world's supply
to-day. There seems to be a superabundance of iron oxide in the rocks of
the African mines and in the diamonds themselves, imparting yellow or
brownish tints to the material. The "River" stones seem to have lost
this color to a considerable extent, if they ever had it. Possibly long
extraction with water has removed the very slightly soluble coloring
material. Whatever the cause of their superiority "River" stones have
always been more highly regarded than stones from the volcanic pipes.
Brazil furnished the world's principal supply of diamonds until the
discovery of the African stones in 1867. At present relatively small
numbers of Brazilian stones reach the world's markets. Most of these
come from the great Bahia district (discovered in 1844) rather than from
the older mines of Brazil. The present Brazilian stones average of small
size. They are, however, of very good quality as a rule. A few green
stones are found in Brazil and these may be of an absinthe-green or of a
pistachio-green tint.
AUSTRALIAN AND AMERICAN SOURCES. While a few diamonds now come on the
market from New South Wales, and while an occasional stone is found in
the United States (usually in glacial drift in the north central States,
or in volcanic material somewhat resembling that of South Africa in
Arkansas) yet the world's output now comes almost entirely from South
Africa and mainly from the enormous volcanic pipes of the Kimberly
district and those of the Premier Co. in the Transvaal.
SOUTH AFRICAN DIAMONDS. The nature of the occurrence of diamond in the
"pipes" of South Africa is so well known to all who deal in diamonds
to-day that but little space need be devoted to it. The "blue ground,"
as the rock in which the diamonds are found is called, seems to have
been forced up from below, perhaps as the material of a mud volcano,
bringing with it the diamonds, garnets, zircons, and the fifty or more
other minerals that have been found in the blue ground. The fragmentary
character of some of these minerals would indicate that the blue ground
was not their original matrix. How the diamonds originally crystallized
and where, is still probably a matter for further speculation.
While at first the mines were worked, like quarries, from the surface,
and while the great Premier mine is still so worked, most of the present
mines are worked by sinking shafts in the native rock outside of the
blue ground and then tunneling into the diamond-bearing rock laterally,
removing it to the surface, allowing it to weather on the "floors" until
it crumbles, then crushing and washing it and concentrating the heavy
minerals by gravity methods. Large diamonds are then picked out of the
concentrates by hand and small ones and fragments are removed by the
"greasers," which are shaking tables heavily smeared with grease over
which the concentrates are washed and to which diamond alone, of all the
minerals in the concentrate, sticks. The grease is periodically removed
and melted, and the diamonds secured. The grease can then be used again.
German South West Africa furnishes a considerable output of very small
diamonds, which are found in dry sand far from any present rivers. These
diamonds cut to splendid white melee and the output is large enough to
make some difference in the relative price of small stones as compared
to large ones. The South West African field seldom yields a stone that
will afford a finished quarter-carat diamond.
RUBIES. Passing on to the occurrence of the _corundum_ gems we will
consider first the _ruby_. Most fine rubies come from Burmah. The
district in which they are found is near Mogok. Practically all the fine
pigeon-blood rubies come from this district. The fashion for red stones
being for the time little in evidence rubies are not now in great
demand. This cessation of demand can hardly be laid to the competition
of the scientific ruby, for the sapphire is now very much in vogue, yet
scientific sapphires resemble the natural ones even more closely than do
the rubies.
Siam furnishes a considerable number of dark garnet-like rubies. These
do not command high prices. They are, however, sometimes very beautiful,
especially when well cut for brilliancy, and when in a strong light.
Ceylon furnishes a few rubies and a few red corundums have been found in
North Carolina.
The Burmese rubies appear to have been formed in a limestone matrix, but
most of those obtained are gotten from the stream beds, where they have
been carried by water after weathering out from the mother rock.
The rubies of Ceylon, too, probably originated in a limestone matrix,
but are sought in stream gravels.
SAPPHIRES. Fine blue sapphires originate in Siam in larger numbers than
in any other locality. Kashmir, in India, also supplies splendid
specimens of large size. Ceylon, too, furnishes a good deal of sapphire,
but mostly of a lighter color than the Kashmir sapphire. The Ceylon
sapphires are found in the streams, but originate in rock of igneous
origin.
Montana furnishes considerable quantities of sapphire, some of which is
of very good color. It is, of course, as good as the Oriental if of
equal color, being of the same material. The better colored sapphire
from Montana is mined from the rock. Most of the sapphires found in the
river gravels near Helena, Mont., are greenish blue or of other colors,
and not of fine blue.
Queensland and Victoria in Australia supply considerable quantities of
sapphire. When blue the Australian sapphire is usually too dark to be
very valuable. The golden and other "fancy" sapphires of the trade come
largely from the Ceylon gravels. Siam yields silky brown stones and some
fine green ones. Some of the Australian sapphires when cut in certain
directions yield green stones.
CHRYSOBERYL. Chrysoberyl of the variety Alexandrite now comes mainly
from Ceylon, although formerly from the Ural Mountains.
The cat's-eyes also come chiefly from Ceylon.
The yellowish-green chrysoberyls (which jewelers sometimes call
chrysolite) come both from Ceylon and from Brazil. They are frequently
found in papers of "fancy sapphires" or "fancy color stones," so called.
SPINEL. Spinels are found along with ruby in Burmah and in Siam and they
also occur in the gem gravels of Ceylon. Limestone is the usual matrix
of spinel, although it is more often mined in gravels resulting from the
weathering of the matrix.
TOPAZ. True topaz, of wine-yellow color, comes mostly from Brazil.
Ceylon also furnishes yellow topaz. Asiatic Russia furnishes fine large
blue or blue-green crystals resembling aquamarine in appearance. Most of
the topaz found in other localities is pale or colorless. Several of our
western States, notably Utah, Colorado, and California, furnish
colorless topaz. Mexico and Japan also produce it. It is seldom cut,
for, while producing a rather brilliant stone, it has little "fire" and
is therefore not very attractive.
EMERALD AND AQUAMARINE. Beryl of the emerald variety is exceedingly
scarce in the earth. Most of the best emerald comes from Colombia, South
America. Large crystals of paler color come from the Urals.
Like ruby and spinel, emerald usually originates in limestone. One is
tempted to suspect that these stones are of aqueous origin and that
sapphires, and beryl, other than emerald, are more likely of igneous
origin.
Beryls of the aquamarine type occur in many places, but usually of too
pale a tint, or too imperfect, to be worthy of cutting. Fine gem beryl
of blue and blue-green tints comes from Siberia and from several places
in the Ural Mountains on their Asiatic slopes.
The Minas Geraes district of Brazil, famous for all kinds of gem stones,
furnishes most of the aquamarine of commerce. The pegmatite dikes of
Haddam Neck, Conn., of Stoneham, Me., and of San Diego County, Cal.,
have furnished splendid aquamarine and other beryl. These dikes,
according to the geological evidence, are the result of the combined
action of heat and water. Thus both melting and dissolving went on
together and as a result many fine gem minerals of magnificent
crystallization were formed during the subsequent cooling. The longer
the cooling lasted and the more free space for growth the crystals had,
the larger and more perfect they got. The author has himself obtained
finely crystallized aquamarine and tourmaline from the Haddam, Conn.,
locality and the best specimens there occur in "pockets" or cavities in
the coarse granite. Within, these pockets are lined with crystals of
smoky quartz, tourmaline, beryl, and other minerals. Sometimes crystals
occur in mud or clay masses inside the cavities and such crystals,
having been free to grow uninterruptedly in every direction, were
perfect in form, being doubly terminated, and not attached anywhere to
the rock.
Madagascar has in recent years furnished the finest pink beryl, which
has been named Morganite. Yellow beryl (Heliodor) and aquamarine also
occur in Madagascar.
ZIRCON. Zircon comes on the market mainly from Ceylon. It deserves to be
as much esteemed in this country as it is in Ceylon, for its optical
properties are such that it is a very snappy stone. Some of the colors
in which it occurs, such as the golden browns, lend themselves nicely to
the matching of gems and garments, and, with the growth of education in
such matters, jewelers would do well to get better acquainted with the
possibilities of zircon and to introduce it to their customers. The
supply from Ceylon is sufficient to justify popularizing the stone.
Small zircons are found in almost every heavy concentrate, as, for
example, in the concentrates of the diamond mines of South Africa, and
in those of gold placers in many places. The rough stones resemble rough
diamonds in luster and are sometimes mistaken for diamonds.
GARNETS. Garnets of various types are found widely distributed in
nature. Perhaps the Bohemian supply is best known, having furnished a
host of small stones which have usually been rose cut for cluster work
or made into beads. The Bohemian garnets are of the pyrope or fire-red
type. Relatively few large stones of sufficient transparency for cutting
are produced in the Bohemian mines. The so-called "Cape rubies" of the
diamond mines of South Africa are pyrope garnets and some large and fine
ones are found. The "Arizona rubies" are pyrope garnets, and while
seldom of notable size, some are of very fine color, approaching deep
rubies, and the color remains attractive by artificial light.
Almandite garnet, the "almandine" of the jeweler is less abundant than
pyrope, when of gem quality. Ceylon furnishes some and India furnishes
perhaps more. Brazil, from its prolific gem gravels at Minas Novas,
supplies good almandite, and smaller quantities are found in many
different localities.
Hessonite garnet, the cinnamon stone or "hyacinth" (incorrect) of the
trade, comes mainly from Ceylon.
Andradite garnet, of the variety known as demantoid, from its
diamond-like properties, and which is usually sold under the misleading
name "olivine" in the trade, comes from the western slopes of the Ural
Mountains.
TOURMALINE. Gem tourmaline comes from Ceylon, from Madagascar, from the
Ural Mountains, from Brazil, from Maine, from Connecticut, and from
California.
The Ceylon tourmalines are mostly yellow or yellowish green, sometimes
fine olive-green. Those from the Urals may be pink, blue or green.
Brazilian tourmalines are usually green, but sometimes red. In fact in
many localities several colors of tourmaline are usually found together
and it may be that a single crystal will be green in most of its length
but red or pink tipped. Some crystals have a pink core and a green
exterior. The author has found both of the two latter types in the
Haddam, Conn., tourmalines, and on one occasion was surprised to get
back a wine-colored tourmaline from a cutter to whom he had sent a green
crystal. There was but a thin shell of the green material on the
outside of the crystal.
Some of the Madagascar tourmaline is of a fine brownish red, almost as
deep as a light garnet, and much clearer than most garnet.
Would it not be fitting on account of its occurrence in several
localities in the United States, for Americans to use more tourmaline in
their jewels? The quality of some of the tourmalines of Maine, and of
California especially, is not excelled by tourmaline from any other
locality. Some of the Maine tourmaline is of a delightful, slightly
bluish-green tint that almost approaches emerald.
KUNZITE. Spodumene, of the variety kunzite, comes from San Diego County,
California.
QUARTZ GEMS. Coming now to the quartz gems we find amethyst and citrine,
or golden quartz widely distributed so that only the localities that
furnish the better grades of these stones need be mentioned. Siberia and
Uruguay furnish fine amethyst. Brazil also furnishes large quantities
of very good quality.
AMETHYST. The chief charm of the Siberian amethyst lies in its large red
component, which enables it to change from a deep grape-purple by
daylight to a fine red by artificial light that is rich in red rays, and
poor in blue ones. The paler types of amethysts that were once esteemed,
probably for lack of the rich deep variety, become gray in appearance
and much less lovely under artificial light. India furnishes some
amethysts, and papers of "fancy color stones" containing native cut gems
from Ceylon, frequently contain amethysts, but Brazil, Uruguay, and
Siberia furnish the great bulk of the stones that are regarded as choice
to-day.
YELLOW QUARTZ. Citrine or golden quartz comes mainly from Brazil. The
"Spanish topaz" is sometimes the result of heating smoky quartz from
Cordova province in Spain. Our own western mountains furnish
considerable yellow and smoky quartz fit for cutting.
ROSE QUARTZ. Rose quartz of the finest quality comes from South Dakota.
Bavaria, the Ural Mountains, and Paris, Maine, have also furnished it.
AGATE. Agates of the finest types, such as carnelian and sard, come
principally from Brazil and from India.
OPAL. Opals now come most largely from Australia, the Hungarian mines
yielding but few stones at present. The fine black opals of New South
Wales are unsurpassed by any that have ever been found elsewhere. Mexico
furnishes considerable opal, and is notable for its fine "fire opal" or
"cherry opal."
JADE. Most of the jade of the variety nephrite that is obtained to-day
comes from several of the provinces of China or from Siberia or from
Turkestan. A dark-green nephrite comes from New Zealand.
Jade of the jadeite variety, which is harder than nephrite and more
highly valued, is rare. The best specimens come from Upper Burmah. It
is also found in China and in Tibet.
PERIDOT. Peridot, and the brighter olivine or chrysolite, while of the
same mineral species, do not seem to occur together. The darker
bottle-green specimens come from the Island of St. John in the Red Sea.
It is said that many of the finer peridots now available have been recut
from old stones mined many years ago.
Queensland supplies light-green chrysolite, and Arizona a
yellowish-green variety. Light-green stones have been found near the
ruby mines of Upper Burmah.
MOONSTONE. Moonstone comes mainly from Ceylon. The native cut specimens
are sent here and recut, as, when native cut, the direction of the grain
is seldom correct to produce the moonlight effect in symmetrical
fashion. The native cutters apparently try to retain all the size and
weight that is possible, regardless of the effect.
TURQUOISE. Turquoise of the finest blue and most compact texture (and
hence least subject to color change) comes from the province of Khorasan
in Persia. Several of our western states supply turquoise of fair
quality, notably New Mexico, Arizona, Nevada, and California.
LAPIS LAZULI. Lapis Lazuli comes from Afghanistan, from Siberia, and
from South America.
MALACHITE. Malachite is found in many copper mines, but principally in
those of the Ural Mountains.
AZURITE. Azurite is found in the Arizona mines and in Chessy, in France
(hence the name chessylite, sometimes used instead of azurite).
* * * * *
REFERENCES. Students who wish to get a fuller account of the occurrence
of precious stones should run through G. F. Herbert-Smith's _Gem-Stones_
under the different varieties. This work is the most recent authentic
work of a strictly scientific character. Dr. George F. Kunz's _Gems and
Precious Stones of North America_ gives a detailed account of all the
finds in North America up to the time of publication. Many of these are
of course of little commercial importance. The _Mineral Resources of the
United States_ contains annually a long account of the occurrences of
gem materials in this country. A separate pamphlet containing only the
gem portion can be had gratis from the office of the United States
Geological Survey, Washington, D. C.
LESSON XXII
HOW ROUGH PRECIOUS STONES ARE CUT
ROUGH PRECIOUS STONES. John Ruskin, who had the means to acquire some
very fine natural specimens of gem material was of the opinion that man
ought not to tamper with the wonderful crystals of nature, but that
rather they should be admired in the rough. While one can understand
Ruskin's viewpoint, nevertheless the art of man can make use of the
optical properties of transparent minerals, properties no less wonderful
than those exhibited in crystallization, and indeed intimately
associated with the latter, and, by shaping the rough material in
accordance with these optical properties, greatly enhance the beauty of
the gem.
No material illustrates the wonderful improvement that may be brought
about by cutting and polishing better than diamond. In the rough the
diamond is less attractive in appearance than rock crystal. G. F.
Herbert-Smith likens its appearance to that of soda crystals. Another
author likens it to gum arabic. The surface of the rough diamond is
usually ridged by the overlapping of minute layers or strata of the
material so that one cannot look into the clear interior any more than
one can look into a bank, through the prism-glass windows that are so
much used to diffuse the light that enters by means of them. Being thus
of a rough exterior the uncut diamond shows none of the snap and fire
which are developed by proper cutting.
As the diamond perhaps shows more improvement on being cut than any
other stone, and as the art of cutting the diamond is distinct from that
of cutting other precious stones, both in the method of cutting and in
the fact that the workers who cut diamonds cut no other precious stones,
it will be well to consider diamond cutting separately.
Before discussing the methods by which the shaping and polishing are
accomplished let us consider briefly the object that is in view in thus
altering the shape and smoothing the surface of the rough material.
HOW CUTTING INCREASES BRILLIANCY. Primarily the object of cutting a
diamond is to make it more brilliant. So true is this that the usual
form to which diamonds are cut has come to be called the _brilliant_.
The adjective has become a noun. The increased brilliancy is due mainly
to two effects: First, greatly increased reflection of light, and
second, dispersion of light. The reflection is partly external but
principally internal.
Taking up first the internal reflection which is responsible for most of
the white brilliancy of the cut stone we must note that it is a fact
that light that is passing through any transparent material will, upon
arriving at any polished surface, either penetrate and emerge or else it
will be reflected within the material, depending upon the angle at
which the light strikes the surface. For each material there is a
definite angle outside of which light that is passing as above
described, is _totally reflected_ within the material.
[Illustration: FIG. 9.
_AB_ represents the back surface of a piece of diamond.
_CD_ is a line perpendicular to _AB_.
Angle _CDE_ is about 24 degrees.
Dotted line, _FDH_ represents the course taken by a ray of light which
is totally reflected at _D_ in such fashion that angle _FDA_ equals
angle _HDB_.
Any light proceeding towards _AB_ but between _E_ and _C_, would fail to
be totally reflected. Most of it would penetrate _AB_.]
TOTAL REFLECTION. For diamond this _critical angle_, as it is called, is
very nearly 24° from a perpendicular to the surface. If now, we shape a
diamond so that most of the light that enters it from the front falls
upon the first back surface that it meets, at an angle greater than 24°
to a perpendicular to that surface, the light will be totally reflected
within the stone. The angle at which it is reflected will be the same as
that at which it meets the surface. In other words the angles of
incidence and of reflection are equal. See Fig. 9 for an illustration of
this point.
THEORY OF THE "BRILLIANT." In the usual "brilliant" much of the light
that enters through the front surface is thus totally reflected from the
first rear facet that it meets and then proceeds across the stone to be
again totally reflected from the opposite side of the brilliant. This
time the light proceeds toward the top of the stone. See Fig. 10--(From
G. F. Herbert-Smith's _Gem-Stones_).
The angles of the top of a brilliant are purposely made so flat that the
up coming light fails to be totally reflected again and is allowed to
emerge to dazzle the beholder. In the better made brilliants the angle
that the back slope makes with the plane of the girdle is very nearly
41° and the top angle, or angle of the front slope to the plane of the
girdle is about 35°. Such well made brilliants when held up to a bright
light appear almost black--that is, they fail to pass any of the light
through them (except through the tiny culet, which, being parallel to
the table above, passes light that comes straight down to it).
[Illustration: FIG. 10.--COURSE OF THE RAYS OF LIGHT PASSING THROUGH A
BRILLIANT.]
In other words, instead of allowing the light to penetrate them,
well-made brilliants almost totally reflect it back toward its source,
that is, toward the front of the stone. The well-cut diamond is a very
brilliant object, viewed from the front.
We must now consider how the "fire" or prismatic color play is produced,
for it is even more upon the display of fire than upon its pure white
brilliancy that the beauty of a diamond depends.
CAUSE OF "FIRE." As we saw in Lesson X. (which it would be well to
re-read at this time), white light that changes its course from one
transparent medium to another at any but a right angle to the surface
involved, is not only refracted (as we saw in Lesson II.) but is
dispersed, that is, light of different colors is bent by differing
amounts and thus we have a separation of the various colors. If this
takes place as the ray of light leaves the upper surface of a brilliant
the observer upon whose eye the light falls will see either the red, or
the yellow, or the blue, as the case may be, rather than the white light
which entered the stone. If instead, the dispersion takes place as the
light enters the brilliant the various colored rays thus produced will
be totally reflected back to the observer (slightly weakened by
spreading, as compared to the direct or unreflected spectra). Thus
dispersion produces the "fire" in a brilliant.
Other materials than diamond behave similarly, but usually to a much
smaller extent, for few gem materials have so high a refractive power or
so great a dispersive power as diamond.
Having considered the theory of the brilliant we may now take up a study
of the methods by which the exceedingly hard rough diamond is shaped and
polished.
CLEAVING DIAMONDS. If the rough material is of poor shape, or if it has
conspicuous defects in it which prevent its being made into a single
stone, it is cleaved (_i. e._, split along its grain). Hard as it is,
diamond splits readily in certain definite directions (parallel to any
of the triangular faces of the octahedral crystal). The cleaver has to
know the grain of rough diamonds from the external appearance, even
when the crystals, as found, are complicated modifications of the simple
crystal form. He can thus take advantage of the cleavage to speedily
reduce the rough material in size and shape to suit the necessity of the
case. The cleaving is accomplished by making a nick or groove in the
surface of the rough material at the proper point (the stone being held
by a tenacious wax, in the end of a holder, placed upright in a firm
support). A thin steel knife blade is then inserted in the nick and a
sharp light blow struck upon the back of the knife blade. The diamond
then readily splits.
"CUTTING DIAMONDS." The next step is to give the rough material a shape
closely similar to that of the finished brilliant but rough and without
facets. This shaping or "cutting" as it is technically called, is done
by placing the rough stone in the end of a holder by means of a tough
cement and then rotating holder and stone in a lathe-like machine.
Another rough diamond (sometimes a piece of bort, unfit for cutting,
and sometimes a piece of material of good quality which it is necessary
to reduce in size or alter in shape) is cemented into another holder and
held against the surface of the rotating diamond. The holder is steadied
against a firm support. It now becomes a case of "diamond cut diamond,"
each stone wearing away the other and being worn away itself.
The cutting process is fairly rapid and it leaves the stone (which is
reversed to make the opposite side) round in form and with a rounding
top and cone-shaped back. Stones of fancy shape, such as square, or
cushion shape, have to be formed in part by hand rubbing or "bruting" as
it is called.
The facets must now be polished onto the stone. Usually the workers who
cut do not cleave or polish.
"POLISHING" DIAMONDS. The polisher fixes the cut stone firmly in a
metallic holder called a dop, which is cleverly designed to hold the
stone with much of one side of it exposed. The holder is then inverted
so that the stone is beneath and a stout copper wire attached to the
holder is then clamped firmly in a sort of movable vise. The latter is
then placed on the bench in such a position that the diamond rests upon
the surface of a rapidly revolving horizontal iron wheel or "lap" as it
is called. The surface of the latter is "charged" with diamond dust,
that is, diamond dust has been pushed into the metal surface which thus
acts as a support to the dust. The latter wears away the diamond,
producing a flat facet. The lap is kept moistened with oil and from time
to time fresh oil and diamond dust are applied. A speed of about 2,000
rotations per minute is used.
FACETTING. The making of the facets is rather slow work, especially
when, as is usually the case in making the "table" the work has to be
done against one of the "hard points" of the crystal. Great care has to
be taken to place the stone so that the grain lies in a correct
position, for diamond cannot be polished against the grain, nor even
exactly with it, but only obliquely across it. This requirement, as much
as anything, has prevented the use of machines in polishing diamonds.
The table is usually first polished on, then the four top slopes,
dividing the top surface into quarters, then each of the four ridges
thus left, is flattened, making eight facets and finally 32 facets,
exclusive of the table, are made upon the top of the brilliant. The
stone is then reversed and 24 facets, and the culet, polished on the
back. As each facet nears its proper shape the stone is placed upon a
particularly smooth part of the lap and a slight vibratory motion given
to the holder by the hand. This smooths out any lines or grooves that
may have formed because of inequalities of surface of the lap. When
completely facetted the brilliant is finished and requires only to
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