ARTIFICIAL FORMATION OF PRECIOUS STONES.
A clear distinction
must be made between the imitation of a precious stone and its actual
reproduction or formation by artificial methods. In the former case we
simulate the appearance of the natural substance by means of some
product or preparation, which may be (and generally is) widely
different in chemical composition and even in many physical properties.
In the latter case we form the very mineral which Nature has formed,
endowed with all its chemical and physical characters, but not
necessarily produced by processes identical with those of Nature. A few
examples of the true reproduction of precious stones will serve to
explain the distinction pointed out with sufficient exactitude.
Take
the case of the ruby and sapphire, varieties of crystallized alumina
or corundum. If, by the aid of the intense heat of the oxyhydrogen
blowpipe pure alumina, with traces of chromium oxide or other colouring
oxide, be fused, we get a sapphire or ruby glass, having a hardness and
density much less than those belonging to crystals of alumina. But by
prolonging the time of cooling or by producing the alumina from some of
its compounds during the heating, a portion of the product will
crystallise in forms identical with those of the natural stone, and
having the density of 4 and the hardness 9. For some time the specimens
made were small in size and poor in colour and brilliancy, but the
product was identical with native corundum. Now there have been, among
the large numbers of artificially-prepared rubies, some of several
carats in
