ORIGIN, PROPERTIES, CLASSIFICATION, ETC. 2$
marble, or aragonite, constitutes a very desirable material
for some kinds of decorative work, and may rank with the
Mexican onyx.
Artificial Gems. — There is a difference between an artificial
stone and an imitation ; in the latter, there is an entirely
different chemical composition, while an artificial gem can be
manufactured from the same chemical substances, and with
the same physical properties as the natural specimen, and
can be made even to excel the genuine production in brilliancy and play of colors. For instance, the corundum may
be obtained by a chemical process, with the same form of
crystals, and of the same density and hardness as the ruby and
the sapphire, while the artificial spinel cannot be distinguished
from Nature's work, by the eye. The same is true of other
precious stones more especially the compounds of silica.
Artificial rubies are secured by heating alumina for a long
time in a platinum vessel with borax, after which they present
the same crystalline form, hardness, and dichroism as the real
gem. By repeated experiments, chemists have succeeded in
making what were supposed to be artificial diamonds, but with
them, as with other precious stones originating in the laboratory, they are too minute for practical purposes. No artificial
gems are known in _ commerce.
Imitations have been secured with much greater facility
than artificial varieties, and may be produced of any required
size. They are generally made of flint glass and lead, colored
by certain oxides as cobalt, manganese, nickel, copper, iron,
chrome, and some other substances, the composition being
called strass, from the name of the inventor, or paste, whence
the name "paste jewels." Glass jewels are not a modern
invention, for as soon as the secret of making glass was understood, it was employed in imitating precious stones ; in the
