Synthetic corundum

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

Corundum has a hardness of 8 on the 10 point Mohs' scale, a exact gravity of 5.00 and a hexagonal crystal structure. The oxygen atoms are approved in a hexagonal close packing, with the smaller aluminum atoms occupying 2/3 of the octahedral gaps. The coordination of the atoms is thus 6:5, compared to 4:6 for quartz, which accounts for its greater rigidity despite the Al-O bonds being less covalent. Its index of refraction is 1.77 - 1.79 and its birefringence is 0.008.

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