THE ORIGIN OF PRECIOUS STONES.
Though the origin, formation, composition, characteristics and tests of
each stone will be examined in detail when dealing with the stones
seriatim, it is necessary to enquire into those particulars of origin
which are common to all, in order thoroughly to understand why they
differ from other non-metallic and metallic minerals.
At the very commencement we are faced with a subject on which
mineralogists and geologists are by no means in full agreement, and
there seems just ground for considerable divergence of opinion,
according to the line of argument taken. It is a most remarkable fact
that, precious as are certain stones, they do not (with a few
exceptions) contain any of the rarer metals, such as platinum, gold,
etc., or any of their compounds, but are composed entirely of the common
elements and their derivatives, especially of those elements contained
in the upper crust of the earth, and this notwithstanding the fact that
gems are often found deep down in the earth. This is very significant,
and points to the conclusion that these stones were formed by the slow
percolation of water from the surface through the deeper parts of the
earth, carrying with it, in solution or suspension, the chemical
constituents of the earth's upper crust; time and long-continued
pressure, combined with heat or cold, or perhaps both in turn, doing the
rest, as already mentioned.
The moisture falling in dew and rain becomes acidulated with carbonic
acid, CO_{2} (carbon dioxide), from the combustion and decay of organic
matter, vegetation, and other sources, and this moisture is capable of
dissolving certain calcareous substances, which it takes deep into the
earth, till the time comes when it enters perhaps a division-plane in
some rock, or some such cavity, and is unable to get away. The hollow
becomes filled with water, which is slowly more and more charged with
the salts brought down, till saturated; then super-saturated, so that
the salts become precipitated, or perhaps crystallised out, maybe by the
presence of more or other salts, or by a change in temperature. These
crystals then become packed hard by further supplies and pressure, till
eventually, after the lapse of ages, a natural gem is found, _exactly
filling_ the cavity, and is a precious find in many cases.
If now we try to find its analogy in chemistry, and for a moment
consider the curious behaviour of some well-known salts, under different
conditions of temperature, what is taking place underground ceases to be
mysterious and becomes readily intelligible.
Perhaps the best salt for the purpose, and one easy to obtain for
experiment, is the sulphate of sodium--known also as Glauber's Salt.
It is in large, colourless prisms, which may soon be dissolved in about
three parts of water, so long as the water does not exceed 60 deg. F., and
at this temperature a super-saturated solution may easily be made. But
if the water is heated the salt then becomes more and more insoluble as
the temperature increases, till it is completely insoluble.
If a super-saturated solution of this Glauber's Salt is made in a glass,
at ordinary atmospheric temperature, and into this cold solution,
without heating, is dropped a small crystal of the same salt, there will
be caused a rise in temperature, and the whole will then crystallise out
quite suddenly; the water will be absorbed, and the whole will solidify
into a mass which exactly fits the inner contour of the vessel.
We have now formed what _might_ be a precious stone, and no doubt would
be, if continuous pressure could be applied to it for perhaps a few
thousand years; at any rate, the formation of a natural jewel is not
greatly different, and after being subjected for a period, extending to
ages, to the washings of moisture, the contact of its containing bed
(its later matrix), the action of the changes in the temperature of the
earth in its vicinity, it emerges by volcanic eruption, earthquake,
landslip and the like, or is discovered as a rare and valuable specimen
of some simple compound of earth-crust and water, as simple as Glauber's
Salt, or as the pure crystallized carbon.
It is also curious to note that in some cases the stones have not been
caused by aqueous deposit in an already existing hollow, but the aqueous
infusion has acted on a portion of the rock on which it rested,
absorbing the rock, and, as it were, replacing it by its own substance.
This is evidenced in cases where gems have been found encrusted on their
matrix, which latter was being slowly transformed to the character of
the jewel encrusted, or "scabbed" on it.
The character of the matrix is also in a great measure the cause of the
variety of the stone, for it is obvious that the same salt-charged
aqueous solution which undergoes change in and on ironstone would result
in an entirely different product from that resting on or embedded in
silica.
Following out the explanation of the aqueous solution, in which the
earth-crust constituents are secreted, we find that the rarer and more
precious metals do not generally enter into the composition of precious
stones--which fact may advisedly be repeated. It is, of course, to be
expected that beryllium will be found in the emerald, since it is under
the species beryl, and zirconium in zircon; but such instances are the
exception, and we may well wonder at the actions of the infinite powers
of nature, when we reflect that the rarest, costliest and most beautiful
of all precious stones are the simplest in their constituents.
Thus we find the diamond standing unique amongst all gems in being
composed of one element only--carbon--being pure crystallised carbon; a
different form from graphite, it is true, but, nevertheless, pure carbon
and nothing else. Therefore, from its chemical, as well as from its
commercial aspect, the diamond stands alone as the most important of
gems.
The next in simplicity, whilst being the most costly of all, is the
ruby, and with this may be classed the blue sapphire, seeing that their
chemical constituents are exactly the same, the difference being one of
colour only. These have two elements, oxygen and aluminium, which
important constituents appear also in other stones, but this example is
sufficient to prove their simplicity of origin.
Another unique stone is the turquoise, in that it is the only rare gem
essentially containing a great proportion of water, which renders it
easily liable to destruction, as we shall see later. It is a combination
of alumina, water, and phosphoric acid, and is also unique in being the
only known valuable stone containing a phosphate.
Turning to the silica series, we again find a number of gems with two
elements only, silica--an important constituent of the earth's
crust--and oxygen--an important constituent of atmospheric air. In this
group may be mentioned the opal, amethyst, agate, rock-crystal, and the
like, as the best known examples, whilst oxygen appears also mostly in
the form of oxides, in chrysoberyl, spinel, and the like. This silica
group is extremely interesting, for in it, with the exception of the
tourmaline and a few others, the composition of the gems is very simple,
and we find in this group such stones as the chrysolite, several
varieties of topaz, the garnet, emerald, etc., etc.
Malachite and similar stones are more ornamental than precious, though
they come in the category of precious stones. These are the carbonate
series, containing much carbonic acid, and, as may be expected, a
considerable proportion of water in their composition, which water can,
of course, be dispelled by the application of heat, but to the
destruction of the stone.
From all this will be seen how strong is the theory of aqueous
percolation, for, given time and pressure, water charged with
earth-crust constituents appears to be the origin of the formation of
all precious stones; and all the precious stones known have, when
analysed, been found to be almost exclusively composed of
upper-earth-crust constituents; the other compounds which certain stones
contain may, in all cases, be traced to their matrix, or to their
geological or mineralogical situation.
In contradistinction to this, the essentially underground liquids, with
time and pressure, form metallic minerals and mineralise the rocks,
instead of forming gems.
Thus we see that in a different class of minerals--compounds of metals
with the sulphates, such as sulphuric acid and compounds; also those
containing the metallic sulphides; in cases where the metalliferous ores
or the metallic elements enter into composition with the
halogens--bromine, chlorine, fluorine, and iodine--in all these,
precious stones are comparatively common, but the stones of these groups
are invariably those used for decorative or ornamental purposes, and
true "gems" are entirely absent.
It would therefore appear that though metallic minerals, as already
mentioned, are formed by the action of essentially _underground_
chemically-charged water--combined with ages of time and long-continued
pressure, rocks and earth being transformed into metalliferous ores by
the same means--precious stones (or that portion of them ranking as
jewels or gems) must on the contrary be wholly, or almost wholly,
composed of _upper_-earth-crust materials, carried deep down by water,
and subjected to the action of the same time and pressure; the simpler
the compound, the more perfect and important the result, as seen in the
diamond, the ruby, and the like.