Grades—Varieties of Quartz
The
varieties include crystalline, crypto-crystalline, and clastic and the
gem varieties belong to the first two classes. Another classification
is transparent, translucent and opaque. The cost of cutting governs the
value of gem quartz.
1. Crystalline varieties
(i) Crystalline varieties.—Rock
crystal—pure, transparent, water clear and well crystallized. It is
used for vases, beads, crystal balls, etc., and has enjoyed great
popularity.
(ii) Amethyst.—Purple
or violet—transparent and well crystallized. Colour varies in intensity
or may be unevenly distributed. Heat treatment may improve the colour.
(iii) Rose quartz.—It
is invariably massive, transparent to translucent with a greasy lustre
and milky appearance. It may also show asterism, which is best seen in
transmitted light—may be cut cabochon, etc.
(iv) Smoky quartz.—Cairngorm—smoky
yellow to dark brown and black. It may be decolorized by heat. It is
slightly dichroic, and is used for brooches, pins and other ornaments.
(v) Tiger's eye.—Pseudomorphous after crocidolite and chatoyant when cut en cabochon.
(vi) Cat's eye.—It
has parallel fibres of asbestos and is greyish, brownish or green in
colour. When cut en cabochon, it shows a chatoyant band of reflected
light on the surface.
Other
varieties include citrine or yellow quartz, milky quartz—transparent
and opaque, saphirine quartz—indigo blue, aventurine quartz with scales
of bright coloured minerals, and sagenite with hairlike rutile
inclusions.
2. Crypto-crystalline varieties
(i) Chalcedony.—Transparent and translucent and includes several varieties: (a) Carnelian—reddish; (b) Chrysophrase—
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