PTER II.

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 sal