Corundum

Corundum is the crystalline form of aluminum 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, oriental topaz if yellow, oriental amethyst if violet, oriental emerald if green, and sapphire if blue. The word "corundum" comes from the Hindu kurand.

Corundum has a hardness of 9 on the 10 point Moh's 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. 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.

Due to corundum's hardness, it is commonly used in machining, from huge machines to sandpaper. Diamond is harder, but much more expensive.