82 A Book of Precious Stones
predominates and in the latter the hexagonal pyramid.
Although
corundum is second to the diamond in point of hardness, it is
approached much more closely by the minerals next below it in the scale
of hardness, than it approaches the eminent and reserved diamond.
Pure
corundum has a high specific gravity ranging from 3.94 to 4.08, and
this great density makes the specific gravity test in distinguishing
it from other stones both easy and important. The differently coloured
varieties have not been proved to vary in this particular. Acids will
not attack corundum nor is it fusible before the blowpipe. Some
specimens when heated in the dark are beautifully phosphorescent.
Corundum, by friction, develops positive electricity, which it retains
for some time. The lustre of corundum and its fire approach these
qualities in the diamond, but the lustre is vitreous instead of
adamantine, although it is very durable. Corundum is optically uniaxial
and strongly doubly refracting, but the dispersion produced is slight
and it is, therefore, incapable of emitting flashes of prismatic
colours like the diamond. Coloured corundum crystals are dichroic and
the deeper the colour
