violet tinge, extraordinary ray colour = light yellowish red. Sapphire
shows blue for ordinary ray and yellowish blue for extraordinary ray.
colour of corundum has a wide range. Red corundum is known as ruby, and
formerly was confused with spinel and garnet. The colour may vary from
deep red to pale pink. The colour of the best ruby is known as '
pigeon's blood red', which has been widely copied from book to book.
For sapphire, the finest colour is a royal blue, a dark sky blue or
Indra Neela in Sanskrit of remarkable purity and evenness of
distribution. The next best colour is the corn-flower blue of Kashmir
sapphires. But the colour may vary from inky blue to sky blue, a pale
greyish blue, velvety blue, and corn-flower blue, which are very much
appreciated. The terms ruby and sapphire connote definite meanings, but
corundum of other colours has been much confused in names. Yellow
sapphire is known as 'oriental topaz'.
When exposed to ultra-violet rays, crystals or cut stones of ruby are
seen to glow with a brilliant red light as if on fire. Hence it was
known as carbuncle. This phenomenon is known as fluorescence. Yellow
corundum and some sapphires give a yellow glow.
Silk' and ' asterism' are seen in native corundum but not in synthetic
gem corundum. In ' asterism' a six rayed or twelve rayed star may be
seen, when examined in light and when the stone is cut en cabochon, or
a dome shaped form, the base of which coincides with the basal plane of
the crystal. Star rubies and star sapphires are also valuable -
The precious forms of corundum, i.e. ruby
and sapphire are valuable and costly gem-stones. The impure forms and
emery are used in industry as abrasive. Due to its hardness it is used
as an abrasive agent for grinding and