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Isotropic Composition Si02.x.H20 (contains 3-10% of water). Impurities also occur, e.g. calcium, iron, magnesium, etc.
Grades.—Precious
opal has a number of varieties. It has a delightful play of colours,
and is cut cabochon to show the property best. The precious varieties
have their own names, e.g. white opals, having a light colour; black
opal—colour varies from dark grey to blue and black types—a valuable
variety; harlequin opal contains patches of colour. Fire opal is
yellow, orange, or red in colour. Opal matrix consists of opal in its
matrix. Besides the above some common opals have no play of colours;
milk opal, resin opal, agate opal, rose opal, hyalite, etc.
Occurrence.—Most
of the opal is the result of the decomposition of silicate minerals
and volcanic glass by hot waters, and the silica thus dissolved is
deposited in cracks and crevices. It is found to be widely distributed,
but precious opal is found in only a few places. Opal was obtained from
Czechoslovakia. Precious opal of good quality is also obtained from
Australia, New South Wales and Queensland. Fire opal comes from Mexico
and good opal from Latah country, Idaho.
In
India opal is found in a number of places, e.g. Ahmednagar in Bombay,
Nagpur in C.P., in Hyderabad near the city with chalcedony, carnelian
and amethyst; in the Vizagapatam district in Madras replacing
felspatbic portions of kodurite; and in Ajmer at the base of the hills
near Srinagar.
Chapter XXII
NEPHRITE OR JADE
Introduction
JADEITE
has been known to' man since very early times. It is a semi-precious
stone of great beauty and very much prized by the Chinese, who consider
it a sacred stone
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