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80 A Book of Precious Stones
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in six-sided prisms and pyramids, the crystals frequently being rough
and rounded; hardness 9; brittle; specific gravity 3.9 and upwards to
4.16; lustre adamantine to vitreous; sometimes the lustre is pearly on
the basal plane; and occasionally there is exhibited a bright,
opalescent, six-rayed star in the direction of the vertical axis. The
colour range is almost unlimited, blue corundum being sapphire. The
strongly coloured varieties are pleochroic. Corundum is sometimes
phosphorescent, with a rich red colour. The red-coloured corundum or
ruby varies from a rose to a deep carmine, the desideratum being a "
pigeon's blood " red, and the same crystal will sometimes reveal
different colours. Like its brother in the noble corundum family, the
ruby is a peer of the realm of precious stones, and second only to the
throne of the sovereign diamond.
In
chemistry, corundum is pure alumina, the oxide of the metal aluminum,
composed of 53.2 per cent, of the metal and 46.8 per cent, of oxygen.
Natural corundum is probably never chemically pure; the inclusions of
foreign elements, sometimes but the merest traces, impart the colour
that makes the gem. When foreign matter is present in large proportion
corundum
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