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148 PRECIOUS STONES
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OPAL
Precious or
noble opal is compact and amorphous, occurring as thin strata within a
matrix. It was known and highly esteemed by the ancients, who called it
" paideros," or " child beautiful as love." It is found in Hungary and
Australia. The common varieties occur in Mexico, Honduras, and the
United States.
It consists of a soluble silica and about ten per cent, water.
Hardness, 5.5 to 6.5; specific gravity, 2 to 2.2.
Lustre subvitreous; translucent in various degrees.
Single refraction (index, 1.48).
It is traversed by microscopic fissures, which decompose the rays of light and produce beautiful prismatic colors.
It
is infusible before the blow-pipe, but heat cracks it, and, driving out
the water, renders it opaque. Sulphuric acid turns it black. It is
almost entirely soluble in a cold solution of caustic potash.
Color,
white, yellow, and brownish yellow body, in which the prismatic colors
by an optical phenomenon appear in various degrees. Color is
temporarily improved by the warmth of the hand or mouth. Occasionally
one is found which, when cut, has a chatoyant colored light similar to
the cat's-eye.
It is usually cut en cabochon, and is also carved to represent flowers, insects, etc.
Symbolizes hope, innocence, purity, October.
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