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PRECIOUS STONES.
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refractive, the indices for yellow light being 1*654 and l-670.
The dispersion is weak, and hence there is but little fire in the stone
when cut. The lustre is vitreous, and the mineral is capable of taking
a high degree of polish. The specific gravity is 2'95 to 3"00. It has a
conchoidal fracture, and is brittle. There is a perfect cleavage
parallel to the prism face, and an imperfect one parallel to the
primary rhombo-hedron. It crystallises in rhombohedral forms, and the
habit is either stoutly prismatic or rhombohedral. In composition it is
a beryllium orthosilicate,
found
in mica-schist with Emerald and Chrysoberyl, at Takovaya, in the Ural
Mountains ; also on the Ilmen Mountains with Amazonstone and Topaz. In
America it is found 16 miles from Pike's Peak, in Colorado ; at Topaz
Butte, with Topaz and Amazonstone in granite ; also at Mount Antero, in
Colorado, with Quartz and Beryl. The chief demand for the gem is in the
countries where it is mostly found—Kussia and America. The colourless
specimens are cut as brilliants, and mounted in an open setting, and
the coloured ones may be treated in the same way if the shade is good.
383. Dioptase.
Dioptase
and Malachite are of interest among gem stones on account of the large
percentage of the common metal copper entering into their composition.
One might almost call Dioptase a copper ore, and certainly Malachite
is. In colour it is emerald-green, but of a deeper tint than the
Emerald itself has ; the lustre is vitreous ; it is transparent to
translucent and shows strong double refraction, the indices being l-667 and 1.723. When heated it turns
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