Aluminium oxide, Al2O3, the hardest naturally
occurring mineral known apart from diamond
(corundum rates 9 of the Mohs' scale); lack
of cleavage also increases its durability.
Gem quality corundum is ruby (red) or
sapphire (any other colour including blue).
Material of lesser quality is used as an
abrasive and for other industrial purposes;
synthetic corundum is also used in industry.
Corundum forms in silica-poor igneous and
metamorphic rocks. It is a constituent of
emery, which is metamorphosed bauxite.
Crystals are barrel-shaped prisms of the
trigonal system.