Corundum << kuh RUHN duhm >> is the second hardest pure mineral. Only diamond is harder. Corundum occurs as transparent nuggets in gravel, and as nontransparent grains and rare transparent crystals in rocks.
Varieties of transparent corundum are polished and used as gemstones. Gemstones from corundum include rubies and sapphires, which may be blue, green, purple, or yellow. The colors of the gemstones are caused by impurities in the corundum. For example, the red of the ruby is caused by the presence of traces of chromium, and the blue of the sapphire by iron and titanium. Gemstone corundum comes mainly from Australia, southeast Africa, Sri Lanka, Myanmar, and India.
Nontransparent corundum is used as an abrasive (grinding, smoothing, and polishing material). Emery, a common abrasive, is a natural mixture of corundum and other minerals. Abrasive quality corundum and emery are mined in Turkey and Greece.
