polishing.
It is applied in a powdered form to paper (emery paper) and is used in
the form of wheels or hones. The powder is used in different grades of
fineness by lapidaries fof grinding gem-stones. Another important
application of corundum due to its hardness is for pivot supports in
delicate instruments as jewel bearings of watches and also for bearings
in scientific instruments.
Grades
(a)
Rubies and sapphires are the gem varieties of corundum. Ruby has
various shades of red, the best colour being the pigeon's blood red, a
shade of red slightly inclined to purple. (6) All precious corundum
other than red is generally called sapphire and the true sapphire is a
royal blue to a corn-flower blue, also known as Kashmir blue. Sapphires
of other colours have separate names, (c) Star rubies and star
sapphires, (d) Common corundum and emery are not useful for gem
purposes. Corundum usually consists of opaque crystals or masses with
dull colours. Emery is a black granular mixture of corundum with
magnetite, hematite, quartz and spinel. Both are used for abrasive
purposes.
Rubies
were known in India from very- ancient times and came chiefly from
Ceylon and Burma. The different varieties were named according to the
colour. The ordinary good ruby is known as 'Padmaraga'. Spinel waff
also included under corundum. The following are the varieties:—
