Corundum
Corundum photo (c) Andrew Alden, licensed to About.com. (fair use policy)
Corundum is an aluminum oxide mineral, the natural form of alumina (Al2O3). It is extremely hard, second only to diamond, and is assigned hardness 9 in the Mohs relative hardness scale. This corundum crystal has a typical tapered shape and hexagonal cross section.
Corundum occurs in rocks that are low in silica, particularly in nepheline syenite, schists altered by alumina-bearing fluids, and altered limestones. It's also found in pegmatites. A fine-grained natural mixture of corundum and magnetite is called emery, which was once a widely used mineral for abrasives.
Pure corundum is a clear mineral. Various impurities give it brown, yellow, red, blue and violet colors. All of these except for red are called sapphire. Red corundum is called ruby. That's why you cannot buy a red sapphire! Corundum gemstones are well known for the property of asterism, in which aligned microscopic inclusions create the appearance of a "star" in a round cabachon-cut stone.
Corundum, in the form of industrial alumina, is an important commodity. Alumina grit is the working ingredient of sandpaper, and sapphire plates and rods are used in many high-tech applications. Most corundum jewelry uses manufactured stones today.
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