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PRECIOUS STONES
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ROCK CRYSTAL
The name is derived from the Greek krystallos, like ice.
It is crystallized quartz, and is found in Brazil, Europe, and the United States.
Crystallization rhombohedral; usually in six-sided prisms variously modified, with pyramidal terminations.
Hardness, 7; specific gravity, 2.65.
Lustre vitreous; transparent to translucent.
Cleavage indistinct; sometimes discovered by heating the crystal and then plunging it into cold water.
Doubly refractive (index, 1.55 and 1.54) ; positively electric by friction; shows phosphorescence in the dark.
It is composed of oxygen, 53; silica, 47.
It
can be melted with the oxyhydrogen blow-pipe. Melts with soda to a
clear glass, and is soluble in fluohydric acid, but is infusible before
the blow-pipe alone.
Colorless.
For gems it is cut brilliant. Largely cut for optical purposes to lenses called " pebbles." Cut in spheres for divination; also to imitate diamonds.
When yellow, it is called citrine or false topaz.
The brown variety is named cairngorm or smoky topaz.
The
black is known as morion, and sagenite or Venus's hair stone is a
variety containing needles of rutile, a brownish-red mineral, which
traverse the stone in the same general direction, but at confused
angles.
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