Corundum

Corundum is the crystalline form of aluminium oxide (Al₂O₃) and one of the rock-forming minerals. Corundum is naturally clear, but can have different colors when impurities are added. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. The word corundum comes from the Hindu kurand.

Corundum has a hardness of 9 on the 10 point Mohs' scale, a specific gravity of 4.00 and a hexagonal crystal structure. The oxygen atoms are arranged in a hexagonal close packing, with the smaller aluminum atoms occupying 2/3 of the octahedral gaps. It's luster is vitreous to adamantine. The coordination of the atoms are thus 6:4, compared to 4:2 for quartz, which accounts for its greater hardness despite the Al-O bonds being less covalent. Its index of refraction is 1.76 - 1.78 and its birefringence is 0.009.

Due to corundum's hardness, it is commonly used in as an abrasive in machining, from huge machines to sandpaper. Emery is an impure and less abrasive variety, with a Mohs hardness of 8.0. Diamond is harder, but much more expensive.