Corundum is a translucent to transparent crystalline (rhombohedral) form of aluminum
oxide (alpha-Al2O3) which comes in many different
hues. It is found in igneous and metamorphic rocks, occurring as pyramidal or prismatic crystals in the rhombohedral class
and as granular masses. The corundum structure has close-packed oxygen with aluminum in octahedral holes. Mineral varieties
of corundum include ruby, sapphire,
and emery. Artificial corundum, or beta-alumina,
is made by calcining bauxite (see calcination).
Mineral Information Institute
Corundum is the hardest natural substance known after diamond and is also chemically inert and resistant to corrosion.
It is used extensively as an abrasive and
in bearings (for example, in motors and
watches). Hardness 9, relative density 4.
AND PLANETARY SCIENCE