elieved to be the native name of the stone (Hindi,
_kurund_; Tamil, _kurundam_; Sanskrit, _kuruvinda_, "ruby"). The finely
coloured, transparent varieties include such gem-stones as the ruby and
sapphire, whilst the impure granular and massive forms are known as
emery. The term corundum is often restricted to the remaining kinds,
i.e. those crystallized and crystalline varieties which are not
sufficiently transparent and brilliant for ornamental purposes, and
which were known to the older mineralogists as "imperfect corundum."
Such varieties were termed by J. Black, in consequence of their
hardness, adamantine spar, but this name is now usually restricted to a
hair-brown corundum, remarkable for a pearly sheen on the basal plane.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

Corundum crystallizes in the hexagonal system. In fig. 1, which is a
form of ruby, the prism a is combined with a hexagonal pyramid n, a
rhombohedron R, and the basal pinacoid C. In fig. 2, which represents a
typical crystal of sapphire, the prism s is associated with the acute
pyramids b, r, and a rhombohedron a. Other crystals show a tabular
habit, consisting usually of the basal pinacoid with a rhombohedron, and
it is notable that this habit is said to be characteristic of corundum
which has consolidated from a fused magma. Corundum has no true
cleavage, but presents parting planes due to the structure of the
crystal, which have been studied by Prof. J. W. Judd.

Next to diamond, corundum is the hardest known mineral. Its hardness is
generally given as 9, but there are slight variations in different
stones, sapphire being rather harder than ruby, and ruby than common
corundum. The colours are very varied, and it is probable that iron is
responsible for many of the tints, though chromium is a possible agent
in certain cases. The transparent varieties are often distinguished as
"Oriental" stones. (See RUBY and SAPPHIRE.) Corundum is used largely for
watch-jewels, and for bearings in electrical apparatus.

The coloured corundums fit for gem-stones come chiefly from Ceylon,
Burma, Siam and Montana. Coarse dull corundum is found in many
localities, and usually has higher commercial value as an abrasive agent
than emery, which is less pure. The coarse corundum, however, is often
partially hydrated or otherwise altered, whereby its hardness is
diminished. In India, where the native lapidaries use corundum-sticks
and rubbers formed of the powde