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CHAPTER VII.
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The Quartz Group:—Rock Crystal, Transparent Quartz, Amethyst, Smoky Quartz,
Cairngorm Stone, Gold Quartz, Rose Quartz, Novaculite, Silicified Coral,
Quartzite, Quartz Inclusions, Thetis Hairstone, Agate, Jasper,
Silicified Wood, Opal, Hydrophane, etc.
T
HE quartz group
consists of a large series of substances, which to the eye are very
unlike each other, and pass under a great variety of names, but. they
are all chemically of one substance, namely, silica. The various
colors are evidently due to the presence of metallic oxides,
principally manganese or iron. Quartz may be divided into two groups,
crystalline and cryptocrystalline. The former crystallizes in the
rhombohedral system, generally as six-sided prisms, with a hardness of
7 and a specific gravity for the colorless of 2*65, the specific
gravity of cairngorm and the amethyst being slightly higher.
Following is a list of the crystalline varieties : Amethyst.—Deep purple, bluish violet fading almost into pink.
Asteriated Star-Quartz.—Containing
between layers of the crystals a deposition of substances, so that when
cut en cabo-chon across the prism it exhibits asterism.
Aventurine.—Transparent to opaque, either red or yellow, with iridescent spangles of mica distributed through it.
Cairngorm.—Transparent, smoky gray, yellow, yellowish brown, and brown.
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