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Ch. 20: Digests

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160
PRECIOUS STONES
SPINEL
At one time thought to be a ruby. It is found in India, Burmah, Ceylon, Afghanistan, Tartary, North America, Sweden, Bohemia, and Australia.
Crystallization isometric; generally octahedron; often found as rolled crystals.
Hardness, 7.5 to 8; specific gravity, 3.5 to 3.7.
Lustre vitreous; transparent.
Cleavage parallel to faces of octahedron, but difficult; fracture conchoidal or uneven.
Single refraction (index, 1.72) ; slightly electric by fric­tion, but not by heat.
Composition: alumina, 72; magnesia, 28; and variable quantities of metallic oxides.
Under the blow-pipe the color changes, but the original color returns as it cools; melts with borax or salts of phos­phorus with difficulty, into a glass; is not attacked by acids.
Color, red, pale and dark violet, indigo blue, purple, light to blackish green; light reflected from the spinel of any color is pale yellow.
Cut step.
The spinel is designated according to color as follows: scarlet, spinel ruby; rose-red, balas ruby; orange-red, rubi-cella; violet, almandine ruby; green, chlorospinel; black, pleonaste.
The balas ruby shows in the rough a tinge of blue at the angles of the octahedron. It is found chiefly at Badakshan in Tartary.
Pleonaste is black and opaque, unfit for gems.
Hercynite is a black iron-spinel found as rolled pieces with the sapphires of Siam, and known by the diggers as " nin."
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