RUBY AND SAPPHIRE
These stones
theoretically are composed of pure alumina, or aluminum, 53.2, and
oxygen, 46.8. This oxide of aluminum, or alumina, is known in its
natural state as corundum, a name derived from the Hindoo word "
kurand," and the two stones ruby and sapphire are frequently spoken of
as the " corundums." It is called ruby when it is red, and sapphire in
any other color.
Comparatively
little is found in the perfectly crystallized transparent condition
necessary for gem purposes. Quantities of this mineral are unfit for
jewels. This common corundum lacks transparency and color. The
colorless transparent, called white sapphire, is the purest. The
colored transparent varieties contain between one and two per cent,
each of silica and oxide of iron, and it is supposed that the fine
color of the Burmah rubies is due to the presence of a very small
quantity of oxide of chromium. The common corundum contains a much
greater proportion of impurities with the alumina. On account of its
hardness it is useful for mechanical purposes. The best of it is used
for bearings in fine machinery. Jewels for watches have long been made
from it, and it is now used also in the manufacture of electrical
supplies.
Various
names are given to the mineral according to the color. The blood-red is
invariably called ruby; blue to colorless is always known as sapphire;
and all the other colors are usually termed fancy sapphires, but
separate names are often attached to them, as follows : Violet,
Oriental amethyst; green, Oriental emerald; yellow, Oriental topaz;
yellowish red, Oriental hyacinth; light bluish green, Oriental
aquamarine.
Although the crystallization of the different corundums is
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