The gem-stones form a small conventional group of minerals, including
principally the diamond, ruby, sapphire, emerald and opal. Other stones
of less value--such as topaz, spinel, chrysoberyl, chrysolite, zircon
and tourmaline--are sometimes called "fancy stones." Many minerals still
less prized, yet often used as ornamental stones,--like moonstone,
rock-crystal and agate,--occasionally pass under the name of
"semi-precious stones," but this is rather a vague term and may include
the stones of the preceding group. The classification of gem-stones is,
indeed, to some extent a matter of fashion.
Descriptions of the several gem-stones will be found under their
respective headings, and the present article gives only a brief review
of the general characters of the group.
Hardness.
A high degree of hardness is an essential property of a gem-stone, for
however beautiful and brilliant a mineral may be it is useless to the
jeweller if it lack sufficient hardness to withstand the abrasion to
which articles of personal decoration are necessarily subjected. Even if
not definitely scratched, the polished stone becomes dull by wear.
Imitations in paste may be extremely brilliant, but being comparatively
soft they soon lose lustre when rubbed. In the article MINERALOGY it is
explained that the varying degrees of hardness are registered on a
definite scale. The exceptional hardness of the diamond gives it a
supreme position in this scale, and to it the arbitrary value of 10 has
been assigned. The corundum gem-stones (ruby and sapphire), though
greatly inferior in hardness to the diamond, come next, with the value
of 9; and it is notable that the sapphire is usually rather harder than
ruby. Then follows the topaz, which, with spinel and chrysoberyl, has a
hardness of 8; whilst quartz falls a degree lower. Most gem-stones are
harder than quartz, though precious opal, turquoise, moonstone and
sphene are inferior to it in hardness. Those stones which are softer
than quartz have been called by jewellers _demi-dures_. To test the
hardness of a cut stone, one of its sharp edges may be drawn, with firm
pressure, across the smooth surface of a piece of quartz; if it leave a
scratch its hardness must be above 7. The stone is then applied in like
manner to a fragment of topaz, preferably a cleavage-piece, and if it
fail to leave a distinct scratch its hardness is between 7 and 8,
whereas if the topaz be scratched it is above 8. An expert may obtain a
fair idea of hardness by gently passing the stone over a fine steel
file, and observing the feel of the stone and the grating sound which it
emits. If a stone be scratched by a steel knife its hardness is below 6.
The degree of hardness of a precious stone is soon ascertained by the
lapidary when cutting it.
Specific gravity.
Gem-stones differ markedly among themselves in density or specific
weight; and although this is a character which does not directly affect
their value for ornamental purposes, it furnishes by its constancy an
important means of distinguishing one stone from another. Moreover, it
is a character very easily determined and can be applied to cut stones
without injury. The relative weightiness of a stone is called its
specific gravity, and is often abbreviated as S.G. The number given in
the description of a mineral as S.G. shows how many times the stone is
heavier than an equal bulk of the standard with which it is compared,
the standard being distilled water at 4° C. If, for example, the S.G. of
diamond is said to be 3.5 it means that a diamond weighs 3½ times as
much as a mass of water of the same bulk. The various methods of
determining specific gravity are described under DENSITY. The readiest
method of testing precious stones, especially when cut, is to use dense
liquids. Suppose it be required to determine whether a yellow stone be
true topaz or false topaz (quartz), it is merely necessary to drop the
stone into a liquid made up to the specific gravity of about 3; and
since topaz has S.G. of 3.5 it sinks in this medium, but as quartz has
S.G. of only 2.65 it floats. The densest gem-stone is zircon, which may
have S.G. as high as 4.7, whilst the lowest is opal with S.G. 2.2.
Amber, it is true, is lighter still, being scarcely denser than water,
but this substance can hardly be called a gem.
Crystalline form and cleavage.
Although the great majority of precious stones occur crystallized, the
characteristic form is destroyed in cutting. The crystal-forms of the
several stones are noticed under their respective headings, and the
subject is discussed fully under CRYSTALLOGRAPHY. A few substances used
as ornamental stones--like opal, turquoise, obsidian and amber--are
amorphous or without crystalline form; whilst others, like the various
stones of the chalcedony-group, display no obvious crystal-characters,
but are seen under the microscope to possess a crystalline structure.
Gem-stones are frequently found in gravels or other detrital deposits,
where they occur as rolled crystals or fragments of crystals, and in
many cases have been reduced to the form of pebbles. By the
disintegration of the rock which formed the original matrix, its
constituent minerals were set free, and whilst many of them were worn
away by long-continued attrition, the gem-stones survived by virtue of
their superior hardness.
Many crystallized gem-stones exhibit cleavage, or a tendency to split in
definite directions. The lapidary recognizes a "grain" in the stone.
When the cleavage is perfect, as in topaz, it may render the working of
the stone difficult, and produce incipient cracks in the cut gem. Flaws
due to the cleavage planes are called "feathers." The octahedral
cleavage of the diamond is taken advantage of in dressing the stone
before cutting it. The cutting of gem-stones is explained under
LAPIDARY.
Colour.
The beauty and consequent value of gems depend mainly on their colour.
Some stones, it is true, are valued for entire absence of colour, as
diamonds of pure "water." Certain kinds of sapphire and topaz, too, are
"water clear," as also is pure rock-crystal; but in most stones colour
is a prime element of attraction. The colour, however, is not generally
an essential property of the mineral, but is due to the presence of
foreign pigmentary matter, often in very small proportion and in some
cases eluding determination. Thus, corundum when pure is colourless, but
the presence of traces of certain mineral substances imparts to it not
only the red of ruby and the blue of sapphire, but almost every other
colour. The tinctorial matter may be distributed either uniformly
throughout the stone or in regular zones, or in quite irregular patches.
A tourmaline, for instance, may be red at one end of a prismatic crystal
and green at the other extremity, or the colour may be so disposed that
in transverse section the centre will be red and the outer zone green. A
beryl may be yellow and green in the same crystal. Sapphire, again, is
often parti-coloured, one portion of the stone being blue and other
portions white or yellow; and the skilful lapidary, in cutting the
stone, will take advantage of the blue portion. The character of the
pigment is in many cases not definitely known. It by no means follows
that the material capable of imparting a certain tint to glass is
identical with that which naturally colours a stone of the same tint;
thus a glass of sapphire-blue may be obtained by the use of cobalt, yet
cobalt has not been detected in the sapphire. Probably the most common
mineral pigments are compounds of iron, manganese, copper and chromium.
If the colour of the stone be discharged by heat, an organic pigment is
presumably present. Some ornamental stones change their colour, or even
lose it, on exposure to sunlight and air: such is the case with
rose-quartz, chrysoprase and certain kinds of topaz and turquoise.
Exposure to heat alters the colour of some stones so readily that the
change is taken advantage of commercially; thus, sherry-yellow topaz may
be rendered pink, smoky and amethystine quartz may become yellow, and
coloured zircons may be decolorized, so as to resemble diamonds.
The colours of some gem-stones are greatly affected by radioactivity,
and Prof. F. Bordas has found this to be particularly the case with
sapphire. From his experiments he believes that yellow corundum, or
oriental topaz, may have been formed from blue corundum under the
influence of radioactive substances present in the soil in which the
sapphire was embedded. Different shades of colour may be presented by
different stones of the same species; and it was formerly the custom of
lapidaries to regard the darker stones as masculine and the paler as
feminine, a full blue sapphire, for instance, being called a "male
sapphire" and a delicate blue stone a "female sapphire." It is notable
that some stones appear to change colour by candle-light and by most
other artificial means of illumination; some amethysts thus become inky,
and certain sapphires acquire a murky tint, whilst others become
amethystine. For an example of a remarkable change of this character,
see ALEXANDRITE.
Refraction.
As the optical properties of minerals are fully explained under
CRYSTALLOGRAPHY, little need be said here on this subject. The
brilliancy of a cut stone depends on the amount of light reflected from
its faces; and in the form known as the "brilliant" the gem is so cut
that much of the incident light, after entering the stone and suffering
refraction, is totally reflected from the facets at the back. The amount
of light which is thus returned to the eye of the observer will be
greater as the angle of total reflection, or critical angle, is smaller,
but this angle will be small if the refractive power of the stone is
great, so that the brilliancy directly depends on the refractivity. The
diamond has the highest refractive index of any gem-stone (2.42).
Jargoon, or zircon, has also a high index (mean 1.95), and sphene, which
is occasionally cut as a gem, is likewise very notable in this respect.
The index of refraction generally bears a relation to the specific
gravity of the stone, the heaviest gems having the highest indices,
though a few minerals offer exceptions. The refractive index, which is
thus a very important character in the scientific discrimination of
gem-stones, may be conveniently determined, within certain limits, by
means of the refractometer devised by Dr G.F. Herbert Smith. This
instrument is an improved form of the total reflectometer, in which the
refractive power of a given substance is determined by the method of
total reflection. It may be used for indices ranging from 1.300 to
1.775, and may be applied to faceted stones without removal from their
settings.
Dispersion.
The play of prismatic colours exhibited by a cut stone, often known as
its "fire," is due to the decomposition of the white light which enters
the stone, and is returned, by internal reflection, after resolution in
to its coloured components. This decomposition depends on the dispersive
power of the substance. The exceptional beauty of the fiery flashes in
the diamond is due to its high dispersion, in other words, to the
difference between the refractive indices for the red rays and the
violet rays at the extremities of the spectrum. The peculiar lustre
exhibited by the diamond is called adamantine, and is shared to some
extent by certain other stones which have a high refractive index and
high dispersion, such as zircon.
Spectroscopic characters.
The use of the spectroscope may be valuable in discriminating between
certain precious stones. It was shown by Sir A.H. Church that almandine
garnet and zircon when simply viewed through this instrument give, under
proper conditions, characteristic absorption spectra, due to the light
reflected from the stone having penetrated to some extent into the
substance of the mineral and suffered absorption. It is sometimes
useful to examine the behaviour of a stone under the action of the
Röntgen rays.
Dichroism.
A very useful means of discriminating between certain stones is found in
their dichroism, or, to use a more general term, pleochroism. Neither
amorphous minerals, like opal, nor minerals crystallizing in the cubic
system, like spinel and garnet, possess this property; but coloured
minerals which are doubly refracting may show different colours, when
properly examined, in different directions. Occasionally this is so
marked as to be detected by the naked eye, as in iolite or dichroite,
but usually the stone needs to be examined with such an instrument as
Haidinger's dichroscope (see CRYSTALLOGRAPHY). It must be remembered
that in the direction of an optic axis the two images will be of the
same colour in all positions of the instrument, and it is therefore
necessary before reaching a definite conclusion to turn the stone about
and examine it in various directions. The use of the dichroscope is so
simple that it can be applied by any one to the examination of a cut
stone, but there are other means of determining the nature of a stone by
its optical properties available to the mineralogist and more suitably
discussed under CRYSTALLOGRAPHY.
Chemical composition.
In chemical composition the gem-stones present great variety. Diamond is
composed of only a single element; ruby, sapphire and the quartz-group
are oxides; spinel and chrysoberyl may be regarded as aluminates;
turquoise and beryllonite are phosphates; and a great number of
ornamental stones are silicates of greater or less complexity, such as
emerald, topaz, chrysolite, garnet, zircon, tourmaline, kunzite, sphene
and benitoite. In the examination of a cut stone chemical tests are not
available, since they usually involve the partial destruction of the
mineral. The artificial production of certain gems by chemical processes
which yield products identical in composition and physical properties
with the natural stones, is described in the article GEM, ARTIFICIAL.
Doublets and triplets are composite stone, sometimes prepared for
fraudulent purposes. In a doublet a slab of real gem-stone covers the
face of a paste, whilst in a triplet the paste is both faced and backed
by a slice of genuine stone. By the action of a suitable solvent, such
as chloroform or in some cases even hot water, the cement uniting the
pieces gives way and the compound character of the structure is
detected.
Before the chemical composition of gem-stones was understood, their
classification remained vague and unscientific. As the ancients depended
almost entirely on the eye, the colour of the stone naturally became the
chief factor in classification. A variety of stones agreeing roughly in
colour would be grouped together under a common name, widely as they
might differ in other respects. Thus the emerald, the peridot, green
fluorspar, malachite, and certain kinds of quartz and jade seem to have
been united under the general name of [Greek: smaragdos] whilst the
ruby, red spinel and garnet were probably grouped together as
_carbunculus_. In this way minerals radically different were associated
on the ground of what is generally a superficial and accidental
character, and rarely of any classificatory value. On the other hand, a
grouping based only on colour led to several names being in some cases
applied to the same mineral species. Thus the ruby and sapphire are
essentially identical in chemical composition and in all physical
characters, save colour.
Superstitions.
Descriptions of precious stones by ancient writers generally are too
vague for exact diagnosis. The principal classical authorities are
Theophrastus and the elder Pliny. Stones were formerly held in esteem
not only for their beauty and rarity but for the medicinal and magical
powers with which they were reputed to be endowed. Up to comparatively
recent years the toadstone, for example, was worn not for beauty but for
sake of occult virtue; and even at the present day certain stones, like
jade, are valued for a similar reason. Prof. W. Ridgeway has suggested
that jewelry took its origin not, as often supposed, in an innate love
of personal decoration, but rather in the belief that the objects used
possessed magical virtue. Small stones peculiar in colour or shape,
especially those with natural perforations, are usually valued by
uncivilized peoples as amulets. The Orphic poem [Greek: Lithika],
reputed to be of very early though unknown date, is rich in allusions to
the virtues of many of the gem-stones. Many of the medical and other
virtues of precious stones were evidently attributed to them on the
well-known doctrine of signatures. Thus, the blood-red colour of a fine
jasper suggested that the stone would be useful in haemorrhage; a green
jasper would bring fertility to the soil; and the purple wine-colour of
amethyst pointed to its value as a preventive of intoxication. Many of
the superstitions came down to modern times, and even at the present day
the belief in "lucky stones" is by no means extinct.
BIBLIOGRAPHY.--The most comprehensive work on gem-stones is Professor
Max Bauer's _Edelsteinkunde_ (1896), translated, with additions, by
L.J. Spencer under the title _Precious Stones_ (1904). Less detailed
are Professor P. Groth's _Grundriss der Edelsteinkunde_ (1887) and
Professor C. Doelter's _Edelsteinkunde_ (1893). Sir A. H. Church's
_Precious Stones_ (1905), intended as a guide to the collections in
the Victoria and Albert Museum, is a convenient introduction: and
Professor H.A. Miers's Cantor Lectures at the Society of Arts on
_Precious Stones_ (1896) may be studied with advantage. For American
stones, the valuable work of Dr G.F. Kunz, _The Gems and Precious
Stones of N. America_, is a standard authority; and the Annual Reports
of this writer and others, published by the Geological Survey of the
United States in the _Mineral Resources_, form a repertory of valuable
information on precious stones in general. The articles in _The
Mineral Industry_ (founded by R.P. Rothwell) should also be consulted.
See likewise O.C. Farrington, _Gems and Gem Minerals_ (Chicago, 1903).
For optical characters reference should be made to G.F.H. Smith, _The
Herbert Smith Refractometer_ (London, 1907); L. Claremont, _The
Gem-Cutter's Craft_ (London, 1906); W. Goodchild, _Precious Stones_
(London, 1908). (F. W. R.*)