Corundum

Corundum, from an Indian name for the mineral, is the general term for native alumina or aluminium sesquioxide (A1₂O₃). It crystallises in six-sided pyramids and other forms belonging to the Hexagonal system, and is unaltered by acids or before the blowpipe, if alone; but dissolves in borax and gives a blue colour on heating with cobalt-nitrate. It has a vitreous lustre, its hardness is 9, and its specific gravity is 3.9 to 4.1. Transparent varieties are known as sapphires (q.v.), and are the most precious of gems. The colourless form is the lux sapphire; the blue, sapphire; the red, oriental ruby; the violet, oriental amethyst; the opaque grey or brown form is corundum, and the impure black form is emery (q.v.). The precious varieties come mainly from Burmah and Ceylon; but beds of corundum, several feet thick, occur in the Alleghanies.