Corundum's a crystalline form of aluminium oxide (α-) and's one of the rock-forming minerals. It's one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. The name corundum probably derived from the Sanskrit "kuruvinda" meaning "ruby" . In India a pinkish-orange sapphire's called padparadscha and a pink light-red sapphire's called pat… (More on Corundum)