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PRECIOUS STONES
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171
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of
perfect cleavage. Hardness, 7.5, and very brittle; specific gravity, 3.
Lustre vitreous; transparent. Very lustrous, trichroic, and
pyro-electric. Composition: silica, 41; alumina, 35 ; glucina, 18;
water, 6. Before the blow-pipe, with strong heat, it swells and finally
melts to a white enamel. Color, pale straw to green and blue. It is
found in Peru, with topaz in Brazil, and with beryl in the Urals.
Chemically it is closely related to the beryls, but is distinguished
from them by its perfect cleavage, and from the topaz by its very
oblique crystals.
Fluorite,
or fluorspar, is a soft mineral found in various parts of Europe and
the United States. Crystallization mono-metric. Hardness, 4; specific
gravity, 3.1 to 3.2. Cleavage perfect parallel to face of octahedron.
Lustre vitreous ; occasionally transparent, when it is termed false
ruby, emerald, sapphire, amethyst, topaz, etc. Some varieties are
extremely phosphorescent at a low temperature, and give out
various-colored lights at different temperatures. It is composed of
nearly equal parts calcium and fluorine,—calcium, 51.3; fluorine,
48.7. It melts under the blow-pipe and is attacked by acids. Pulverized
and moistened with sulphuric acid, the gas given off corrodes glass.
Some shade of yellow is the most common color; also red, green, purple,
colorless, etc. Rarely rose-red and sky-blue. Fluoric acid is obtained
from it.
Fossil
corals consist of carbonate of lime, and are found at Little Traverse
Bay and in Iowa. They take a high polish, and are set in studs,
sleeve-buttons, etc.
Gadolinite
is found in Norway, Sweden, and Greenland, in oblique rhombic prisms.
It is very compact. Lustre resinous. Cleavage indistinct. Color, deep
velvet-black, green, greenish-gray streak. Hardness, 6.5 to 7; specific
gravity, 4.1 to 4.4. Consists mainly of silica, yttria, glucina,
protoxide of iron, and oxide of lanthanum.
Hematite is a peroxide of iron, consisting of iron, 70; oxygen, 30. Its crystallization is rhombohedral; form gen-
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