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140 PRECIOUS STONES
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EMERALD
Emerald is
the grass-green variety of the beryl, of which the aquamarine,
aquamarine chrysolite (a greenish-yellow variety), and golden beryl are
the others.
The finest are found in the Republic of Colombia. It occurs also in Egypt, Russia, Austria, Australia, and the United States.
The
crystallization is hexagonal; form, stout hexagonal and dihexagonal
prisms, variously modified and without regular terminations.
Hardness, 7.5 to 7.8; specific gravity, 2.65 to 2.75. It is generally somewhat lighter than the aquamarine, about 2.67.
Lustre vitreous; transparent to translucent.
Cleavage in four directions, but perfect only parallel to terminal plane; fracture conchoidal and uneven.
It
is doubly refractive to a slight degree (index, 1.582 and 1.576);
dichroic; twin colors, yellowish, and bluish green; positively electric
by friction.
Composition: silica, 68; alumina, 15 to 20; glucina, etc., 11 to 14. The coloring matter is probably an oxide of chromium.
Before
the blow-pipe it becomes clouded, and fuses on the edges with
difficulty. It melts with borax, and is soluble in salts of phosphorus,
but is not attacked by acids.
It
occurs in all shades of green, some showing a very slight tint of
yellow or blue. The gem color is a clean, dark, velvety green; streak
uncolored.
It is usually cut step, occasionally brilliant on upper half, and en cabochon.
Symbolizes immortality, incorruptibility, May, John.
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