The sapphire is also divided into two kinds, viz., Eastern and Western.
The
Oriental sapphire is a clear corundum, which has all the gradations of
blue. It is found only in India, and especially in Ceylon.
In hardness it is equal, if not inferior, to that of the red corundum.
Its
crystals, like those of other corundums, have the primitive form of a
dodecahedron with triangular faces, but most frequently they are
rounded.
Its
specific gravity is 4-01.' It possesses double refraction in such a
degree as to surpass every other stone, not excepting the diamond.
It
takes its place amongst the translucent and transparent gems, and
often its imperfect crystallization gives it a milky opacity.
Like
the ruby, it is composed of alumina almost pure ; its colorization
being due to oxide of iron. It is remarkable how the same metallic
oxide produces two such different colours in the same stone ; and,
whilst remembering that the ruby darkens at the fire, and the sapphire
tends to becoming colourless, we must attribute this diversity to the
greater or less oxidation experienced by the metal at different
degrees of heat.
The Oriental sapphire, to be considered beautiful, must have a clear blue colour, not too light, and of perfect limpidity.
It
thus appears soft as velvet, and is so charming that it justifies the
simile used by Dante to indicate the clearness and vivacity of the air :
