constant
for corundum. The ruby has a flatter habit, whilst the sapphire has a
taller habit, and a combination of these gives rise to barrel shaped
crystals. The faces are striated and grooved parallel to the horizontal
edges. There is also lamellar twinning and crystals may break easily
along this direction. It is not to be mistaken for cleavage for it is
only parting. This lamellar twinning is perhaps due to earth pressures.
It is also considered the cause of 'silk' in ruby and of asterism in
star stones. Contact twins are rare.
Gem
corundum forms small crystals while corundum may form larger crystals.
The largest corundum crystal known measures 2 ft. 3 in. in height and
weighs 335 lb. Another large crystal in the British Museum weighs 341
lb. The habit of ruby and corundum crystals is related possibly to the
nature and composition of the impurities in the mineral. In hardness
corundum comes next to diamond, but it is exceeded by carborundum and
by some artificially made carbides. The specific gravity of corundum
slightly varies from 4 (3-90-4-10). This variation may be due to the
presence of cavities and impurities. Gem varieties are transparent, but
common corundum is transparent to translucent, or opaque and varies in
colour. The optical characters are those typical of a uniaxial crystal
of negative sign. So basal sections are isotropic and resemble spinel,
but in convergent polarized bight the uniaxial interference figure will
at once mark the two minerals. Other directions will show birefringence
and dichroism. Refractive index w = 1-768, e = 1-760: dispersion
0.018—which is much less than in diamond; hence they do not flash
prismatic colours and are less brilliant and depend on their colour for
their appeal. When viewed in the direction of c axis a deeper colour is
seen, than when viewed through prism faces. So in cutting the table
facet, it is done parallel to the basal plane (c) of the crystal.
Pleochroic colours in ruby are as follows:—Ordinary ray colour = deep
red with
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