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162 PRECIOUS STONES
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TOURMALINE
This is
said to be a corruption of the name by which the stone was known in
Ceylon, when it was first brought to Europe. It is found in the United
States, Ceylon, Brazil, Moravia, Sweden, Burmah, and elsewhere.
Crystallization
rhombohedral, in three-, six-, nine-, and twelve-sided prisms
differently terminated at the opposite ends; generally striated and
channelled vertically; often different colors within and externally,
or one color at one end, another at the other end.
Hardness, 7 to 7.8; brittle; specific gravity, 3 to 3.15.
Lustre vitreous; transparent and translucent to opaque.
Cleavage perfect on the basal plane; fracture uneven.
Double
refraction (index, 1.64 and 1.62) in a high degree; cut in slices it is
used in the polariscope; dichroic; twin colors of red, salmon- and
rose-pink; of green, pistachio, and bluish green; of blue, greenish
gray and indigo-blue; electric by friction; some crystals, by heating
(pyro-electric), become positively electric at one end and negative at
the other.
Fusible
under the blow-pipe with difficulty to a spongy enamel; melts with
borax to transparent glass. Rubellite turns white; indicolite and green
turn black under the blowpipe.
Color,
red (rubellite), blue (indicolite), green (Brazilian emerald),
colorless (achroite), black (schorl) ; also gray, yellow, and brown;
streak uncolored.
Cut step and brilliant.
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