The colours of the Siam Ruby are different, being crimson and brownish-red.
crystals of Corundum, including those of Ruby and Sapphire, are often
ill-shaped and rough, and usually very much rolled. The cleavage is
accompanied by conchoidal and uneven fracture, and by brittleness. The
lustre of Corundum is vitreous, but sometimes pearly on the basal
planes, and the crystals, when properly cut, occasionally exhibit a
bright opalescent star of six rays in the direction of the principal
axis. Such crystals form the Star Stones, to be noticed in a subsequent chapter.
refractive index of Corundum is 177, and therefore higher than that of
glass ; hence the great brilliancy of the Corundum gem-stones, when
properly cut and polished. Sir W. Crookes has shown that the Ruby, when
exposed to electric discharge in high vacuo, phosphoresces with a
brilliant red glow.
varieties of Corundum can be scratched by the Diamond, but by no other
mineral. The extreme hardness of Corundum has suggested its
mineralogical name of Adamantine Spar; and it seems likely that the Adamas of early Greek writers was not the true Diamond, but merely a form of Corundum.
Corundum is a mineral which, in its various forms, enjoys a fairly wide
geographical distribution, it is remarkable that the fine red varieties
are extremely rare and restricted in their occurrence. The localities
yielding the Rubies of commerce are indeed practically limited to
Burma, Siam and Ceylon. Even of these localities, it is only Burma that
has acquired celebrity for the favourite tint, the true pigeon's-blood
colour, which always obtains the highest price irr the market. The
Rubies of Siam are generally too dark, and those of Ceylon too pale.