1. Talc.
2. Gypsum.
3. Calcite.
4. Fluor.
5. Apatite. / Sphene.
\ Opal.
6. Feldspar. / Diopside.
\ Moonstone.
/ Epidote.
| Idocrase.
| Peridote.
\ Axinite.
7. Quartz. / Quartz.
| Tourmaline.
| Cordierite.
\ Garnet.
/ Andalusite.
| Zircon.
| Emerald.
\ Phenacite.
8. Topaz. / Spinel.
| Topaz.
\ Chrysoberyl.
9. Corundum. / Ruby.
\ Sapphire.
10. Diamond. Diamond.
Among precious stones diamond stands out pre-eminent as the hardest of
all known substances. Ruby and sapphire are scratched by diamond
alone, while chrysoberyl, topaz and spinel scratch all the remaining
stones, although they do themselves yield to the scratch of ruby and
sapphire. The hardness is a character still generally utilized by the
expert when he is in doubt; in experienced hands it has some value. By
long practice it is possible to form a very close estimate of the
hardness of a given stone, and that often, not by the scratch of the
other minerals in the scale, but by the feel of the stone against a
file; the resistance offered by the stone to the file is taken as a
measure of its hardness. It is not a character capable of any accurate
measurement, neither is it to be recommended for use by inexperienced
persons.
I hope to show, as I go on, that we have now accurate methods of
testing at our disposal which render the trial of hardness quite
unnecessary. But, none the less, the character is one of great
importance, as investing the stone with durability. All the precious
stones, except moonstone, opal and sphene, have at least the hardness
of quartz, and can barely be scratched by metals, even by hard steel.
[Illustration: FIG. 1.--TOTAL REFLECTION OF LIGHT IN GLASS.]
Take next the quality of brilliancy. This depends upon two
things--first, the manner in which rays of light are affected when
they enter or leave the stone, and, secondly, the manner in which this
action can be intensified by the art of the lapidary.
When light passes from one transparent substance to another it is bent
or refracted, as every one knows from the bent appearance of a stick
plunged into water. Consider, now, a ray of light falling upon the
surface of a transparent stone; a portion of the light is reflected,
but a portion enters the stone. In passing from air into the stone it
is refracted inward. When, on the other hand, it passes from a
transparent stone into air, its course is reversed and the emerging
ray is refracted outward or toward the surface. It is, however, with
the emerging as with the entering light, the beam is subdivided, only
a portion is refracted out, another portion of the light is reflected
within the stone.
Consider next successive rays within a piece of glass or a stone which
are about to emerge with different inclinations. (See Fig. 1.) As
their course approaches more nearly to the surface, so will the
emerging rays issue more nearly along the surface of the stone; but
the obliquity of the emerging rays increases much more rapidly than
that of the internal rays, until for one ray in the series the
direction of the light (C in the figure) refracted out coincides with
that surface. What, then, will happen to the light within the stone,
which falls still more obliquely? It cannot be refracted out, and, as
a fact, it is entirely reflected within the stone. Imagine, then, how
much greater is the brilliancy of the beam of light, c, e, d, which is
completely reflected, than that of the intermediate portion of the
reflected light, a, b, c, which has lost a large part of its rays by
refraction. The difference is easily seen by looking at a glass of
water held above the head; the brilliant silvery appearance of the
surface, when viewed obliquely, is due to total reflection. The light,
c, d, e, is said to have been totally reflected; and half the angle
between C and c is called the "angle of total reflection." This angle
depends upon the refractive power of the stone. The angle of total
reflection for diamond is about 25°; in no other stone is the
corresponding angle less than 30°; for most of them it is much
greater; while for heavy glass it is about 40°. Light striking the
internal surface more obliquely is reflected without losing any of its
rays by refraction.
[Illustration: FIG. 2.--TOTAL REFLECTION OF LIGHT WITHIN A BRILLIANT.]
It is very clear, then, that of the light traveling in directions
within a diamond, a far larger proportion is internally reflected than
is the case with any other stone. We shall see presently that it is
this property which gives the diamond its consummate brilliancy.
Another effect produced by refraction is, as every one knows, the
separation of ordinary light into rays of different colors--it is seen
in any prism of glass. This property is known as the "dispersion" of
light; and a stone which possesses great dispersion will exhibit a
beautiful play of spectral colors--will exhibit a high degree of what
is called fire. In this respect again the diamond is pre-eminent; its
dispersion is nearly twice as great as that of other stones.
All these optical properties are beautifully shown by those unworked
jewels of which the smooth facets have been produced by nature; I mean
the crystals of the various minerals. The beauty of natural crystals
of transparent minerals is largely due to the optical effects which I
have just been describing.
The beautiful specimens of rock crystal, calc spar, topaz, emerald,
and other stones which adorn mineral collections are sufficient
evidence of these properties. But it is very certain that natural
crystals, although they possess a beauty of form which is all their
own, are not by a long way so brilliant as the faceted stones which
are cut from them by the art of the lapidary; that a natural diamond
is not so lustrous as a faceted brilliant.
In fact, many of the finest gem stones present a very mean and sordid
aspect before they have passed through the hands of the lapidary; one
has only to compare the dull and unattractive appearance of a parcel
of rough rubies, sapphires or rough diamonds with the finished jewels
displayed in the jewelers' windows to see how much these owe to the
lapidary's art.
In recutting the Koh-i-noor it was thought advisable to spend £8,000
on the process and to reduce its weight from 186 to 106 carats. When
the great Pitt diamond was cut, its weight was reduced from 410 carats
to 137; and the fragments and dust removed were valued at £8,000; but
the extent to which the stone was improved is indicated in the fact
that having been purchased for £20,000, it was after cutting sold for
£135,000.
To understand how the cutting of a precious stone adds to its
brilliancy, we have only to trace the course of the rays within the
stone, and consider how it can best be faceted in order that the light
which enters in various directions on the upper side, or crown, may be
reflected internally from facet to facet on the under side of the
stone with as little loss as possible, and may be thrown out from the
front of the stone. For this purpose the facets must be so arranged
that as much of the light as possible within the crystal shall meet
the facets at an inclination exceeding the angle of total reflection.
A brilliant with its 58 facets is one of the forms which experience
has shown to be best adapted for the purpose. How little of the light
gets through a stone so faceted, and, therefore, how much of it is
totally reflected internally, is easily shown by holding the stone in
a strong beam of light; first so that the light is so reflected, and
then so that the light shall, if possible, be transmitted. In the
latter case, the stone merely throws a dark shadow, indicating that
little light, if any, has passed through it.
A faceted stone is always cut from a single crystal, and not from an
ordinary lump of the mineral, which is generally a mass of crystals.
The chief reason why jewels are cut from natural crystals is that
these, by virtue of their crystalline nature, are remarkably
homogeneous, and therefore clear and limpid when free from cracks and
flaws. A stone which is not homogeneous can never have the purity and
limpid brilliancy of a single crystal, for at every point of contact
of one part with another reflection takes place. Among minerals used
as precious stones which are not crystals may be mentioned the opal.
The opal probably owes its peculiar beauty to the very fact that it is
filled with minute cracks or cavities, each of which contributes some
tint of color by reason of its extreme thinness, just as the colors of
the soap bubble are due to the thinness of its film.
Or take the agate. Here the stone consists of layers of different
materials differently colored. Its beauty is of a different nature
from that of clear crystals, which it can never rival in brilliancy.
Stones like the agate are generally classed apart as semi-precious
stones, and their interest depends upon beauty of structure or color,
or possibly to a large extent upon their rarity. The turquois, for
example, is a very rare stone, which is apparently absolutely
uncrystallized, but possesses great beauty of color, and is therefore
much prized. The same is true of carnelian. On the present occasion we
are not concerned with those opaque or curiously structured minerals
whose beauty resides almost solely in their color.
Those who have had no practical acquaintance with minerals have little
idea how variable and accidental are their colors. They may scarcely
realize that the ruby and the sapphire are the same mineral, and that
this mineral also occurs, and is used in jewelry, absolutely
colorless, when it is known as lux sapphire, green as the so-called
Oriental emerald, and yellow as the so-called Oriental topaz; that
topaz itself may be yellow, brown, blue, or colorless; that zircons
range from colorless through almost all conceivable shades of brown
and green, and that even diamond has been found green, red and blue.
When we come to consider the properties by which precious stones are
recognized, I shall say little or nothing about color, for it is of
little value as a criterion. There are, for example, certain red
stones which the most skillful experts cannot by their color alone
refer with certainty to ruby, garnet or spinel. It might be expected
that a noteworthy difference in chemical composition would accompany
this difference of color, or that the pigment could be ascertained by
analysis. In reality this is scarcely ever the case. It is fairly
certain that the emerald owes its color to the presence of chromium,
but the variation in the analyses of precious stones cannot generally
be attributed to anything indicated by the variation of color.
The chemical composition, though of great general importance in
mineralogy, is of little practical value in the discrimination of
precious stones, since it is usually impossible to sacrifice a
sufficient quantity for chemical analysis. If we are dealing with a
faceted stone, not even the smallest portion can be utilized, for fear
of injuring it.
There is, however, one remarkable optical property, which is
ultimately related to the chemical composition. As is well known, many
substances possess the property of absorbing certain rays of light.
When the solar spectrum produced by admitting ordinary daylight
through a slit, and transmitting it through a prism, is passed through
the glowing vapor of certain substances, particular rays of light are
absorbed, and their absence from the emerging fight is manifested by
corresponding dark bands in the spectrum. The instrument by which the
observations are made is the spectroscope. It is well known to most
people that the solar spectrum itself contains certain dark bands of
this sort, which are produced by vapors that can be identified by the
position of the bands in the spectrum; and thus it is possible to
ascertain something regarding the chemical constitution of the sun and
certain of the heavenly bodies. Now, a precisely similar effect is
produced by certain elements if present in a mineral, by merely
transmitting the light through a piece of it. Thus, transparent
minerals which contain the rare element didymium betray the presence
of that element as soon as they are viewed through a spectroscope by
ordinary daylight; the spectrum is seen to be traversed by black bands
in the green, which are quite characteristic.
Among gem stones there are two which possess this curious property.
One is the variety of red garnet known as almandine, and the other is
the jargoon. The almandine produces characteristic bands in the green
and the jargoon in the red, green and blue portion of the spectrum. To
see these remarkable absorption spectra, to which attention was first
called, I think, by my friend, Prof. Church, it is not necessary to
look through the stone, it is quite sufficient to place it in a strong
light, and look at it through an ordinary pocket spectroscope; the
light which enters the instrument consists largely of rays which have
penetrated the stone, and been reflected from the facets at the back.
These rays produce the absorption spectrum. In this way we are enabled
to identify a jargoon or an almandine merely by looking at it. There
is no test so simple or so easy of application. It is curious that the
almandine, or iron aluminum garnet, is the only garnet which presents
an absorptive spectrum, and it is not yet certain to what element the
bands are due. In the case of jargoon, they are supposed to be caused
by the presence of some uranium compound in the mineral. All the
almandine garnets which I have examined, and nearly all the jargoons,
show these characteristic absorption spectra.
[Illustration: FIG. 3. Graph of Specific gravity vs Refractive index
of gems]
By way of summary, I have thought it desirable to indicate the general
characters of precious stones in a diagram, which exhibits some of
their relationships and also some of their differences in a graphic
manner.
Opal, which is a comparatively light mineral, has a low refractive
power; zircon or jargoon is a heavy mineral, and has a high refractive
power. Let now the refractive power of any mineral (as measured by its
refractive index for yellow light) be represented by a corresponding
length set off from left to right, and let its density (as measured by
its specific gravity) be represented by a corresponding length
measured downward. Fixing in this way a point corresponding to opal,
and another representing the character of zircon, draw a straight line
from the one to the other. It will then be found that the points
which, by their position on the diagram, represent the specific
gravity and refractive index of the various minerals will be very
nearly upon this line; that is to say, as the refractive index of
precious stones increases, so also does their density, and the two
increase together in a remarkably regular manner.
It appears from this table that those minerals which, by their high
refractive power, possess the greatest brilliancy, possess also the
highest specific gravity or weightiness; that the precious stones are
therefore all heavy minerals. There is also a rough general
correspondence between these characters and the hardness of the
stones; the brilliant heavy minerals are also generally speaking hard.
Two remarkable exceptions display themselves. Sphene lies far to the
right of the position which it should occupy according to its specific
gravity; it possesses an extraordinarily high refractive index, and
is, therefore, an extremely brilliant gem stone. On the other hand, a
glance at the scale of hardness shows that it is, unfortunately, one
of the softest of the possible gem stones, and that in this respect it
is not very well fitted for jewelry.
Diamond is still more remarkable; its refractive index places it at
the extreme right of the diagram, with a refractive power, and
therefore a brilliancy, greater than that of any other stone; at the
same time its hardness exceeds that of any mineral, and this
combination of qualities renders it the chief among gem stones,
unequaled for brilliancy and durability, although not a heavy mineral.
Moreover, in dispersion, and therefore in fire, it stands alone.
Minerals which are heavier than zircon, such as the metallic sulphides
and iron glance, are unsuitable for gem stones, since they are nearly
opaque, but they follow the same law, and possess a refractive power
still greater than that of zircon or even diamond.
There is one other stone which is exceptional, but in less degree and
in the other direction, namely, topaz, whose refractive index is not
1.7, as it should be by its position on the line due to the specific
gravity, but 1.62; the point corresponding to topaz must therefore be
placed a short distance to the left of the line. It is curious that
these three exceptional stones lie on the same horizonal line, having
all the same specific gravity, 3.5.
In mentioning the specific gravity I have introduced a property which
is not essential to win esteem for a precious stone, but one which is
of great value in its identification.
We have next then to consider those properties by which precious
stones may in practice be most readily recognized. The table shows
very clearly that specific gravity is one such property. The meaning
of specific gravity is easily explained. A piece of tourmaline of any
size weighs three times as much as an equal volume of pure water at 4°
C., the specific gravity of tourmaline is therefore said to be 3; a
piece of almandine garnet of any size weighs four times as much as an
equal volume of water under the same conditions, and the specific
gravity of garnet is therefore 4.
Now any substance immersed in water loses in weight by an amount
exactly equal to that of the water displaced. Hence, to ascertain the
specific gravity it is only necessary to suspend the stone by a fine
thread to the beam of a balance and weigh it first in air, and then
immersed in water. The first weighing gives the weight of the stone
itself, the difference between the first weighing and the second gives
the weight of the displaced water; hence the specific gravity is found
at once by dividing the weight of the stone by this difference. For
very small stones, where the weights concerned are slight, it is
necessary to use a refined chemical balance. But for ordinary stones a
well made Westphal balance is sufficient.
The Westphal balance is constructed on the principle of the common
steelyard. At one end of the beam is a counterweight, at the other end
the stone is suspended; the beam is divided into ten equal parts. A
weight can be suspended on the beam, and its action, of course, varies
with its position on the beam; at the tenth division from the center
it has a value ten times as great as at the first division.
The specific gravity is then found as follows: First, counterpoise the
counterweight. Let this require a weight, A, on the right hand side of
the beam. Next, find the weight necessary to restore equilibrium when
the stone is suspended from the beam. Let this be B. Then A-B is the
weight of the stone in air. Next raise the vessel of distilled water
below the stone until it is immersed. If C be the weight now required
to restore equilibrium, C-B is the loss of weight in water, and,
finally, the specific gravity is (A-B)/(C-B).
This process is known as "hydrostatic weighing," and can be applied to
any stone, except such as are very small. Great precautions must be
taken, in order to determine the specific gravity with accuracy.
Especially it is necessary to free the stone from all adhering bubbles
of air. For this reason the process of hydrostatic weighing is a
somewhat laborious one.
Now, in order to identify a mineral, it ought to be unnecessary to
determine exactly the specific gravity, provided that means can be
devised for showing that its specific gravity is the same as that of
some known substance. For purposes of identification, a comparative
method is often quite as efficacious, and much more easy than actual
measurement. This may now be done by means of certain heavy liquids.
Wood floats in water because it is lighter than water; iron sinks
because it is heavier; but a substance which possessed exactly the
specific gravity of water would neither float nor sink, but would
remain suspended in the water like a balloon in midair. Taken, then, a
liquid which is heavy--the most convenient is methylene iodide, whose
specific gravity is 3.3--a fragment of zircon will sink in this, and a
fragment of tourmaline will float, but a fragment of the mineral
augite, whose specific gravity is also 3.3, will remain exactly
suspended.
This liquid, then, enables one to say with certainty whether a given
stone has a specific gravity greater or less than 3.3; in the one case
it will sink, in the other it will float.
But methylene iodide further possesses the valuable property of mixing
easily with benzene, which is a very light liquid. Every drop of
benzene added reduces the specific gravity of the mixture, which can
thus easily be made to range between that of chrysolite and that of
opal.
To identify any one of the stones which lie between those limits on
the diagram, it is only necessary to drop it into a test tube or small
vessel containing methylene iodide--the stone will float--benzene is
added drop by drop, the mixture being kept well stirred until a point
is reached at which the stone neither sinks nor floats. Then different
fragments of mineral possessing specific gravities between 3.3 and 2.5
are taken in order of increasing density and dropped into the liquid;
the stone under examination possesses a specific gravity between that
of the last which floated and the first which sinks, and the limits
may, if necessary, be further narrowed by comparing it with other
mineral fragments of known density intermediate between those two. One
great advantage of this method is that the size of the fragment does
not affect the result; a minute fragment only just large enough to be
visible is equally convenient; in fact, more convenient than a larger
one.
If a stone in the rough is under examination, a minute chip can easily
be taken from it, and used for the experiment in the most satisfactory
manner. The method is, moreover, extremely sensitive; a mere drop of
benzene added to a considerable volume of the liquid is sufficient to
send to the bottom a stone which was previously floating.
So much for stones whose density is less than that of chrysolite. As
regards the denser minerals, it was until a short time back impossible
to test them by any such method; they all sank in the heaviest liquid
available. But now, thanks to the fortunate discovery by Dr. Retgers
of the remarkable properties of thallium silver nitrate, all the known
gem stones may be distinguished by a similar process.
This salt, which may be prepared by fusing together in equal molecular
proportions nitrate of silver and nitrate of thallium, possesses the
remarkable property of fusing at a temperature far below that of
either of its constituents, and well below that of boiling water,
while at the same time the fused salt possesses a specific gravity
greater than that of zircon. The salt fuses at 75° C. to a clear
colorless liquid in which zircon just floats; it further possesses the
useful property of being miscible in all proportions with water, so
that the specific gravity can be reduced to any desired extent by
adding water, just as that of methylene iodide, was reduced by adding
benzene. The substance can be kept liquid by maintaining it at a
temperature above 75° C., and this may easily be done by immersing the
vessel in which it is contained in water heated to near the boiling
point.
In these two liquids then we have the means of producing a liquid of
any required density for the discrimination of gem stones, since we
can obtain from one or the other a liquid in which any precious stone
will be exactly suspended.
The nitrate might be used by itself to include the whole series, but
it is more convenient to use the methylene iodide when possible, both
because it can be employed at ordinary temperatures and because it is
cheaper than the nitrate.
Both substances darken on exposure to light, and should be both kept
and used in the dark as far as possible: they are easily freed from
the liquid employed to dilute them. The benzene readily evaporates
spontaneously from the methylene iodide, and the water can be driven
off from the diluted thallium silver nitrate by boiling.
(To be continued.)
* * * * *