Corundum is a very hard mineral, at 9 hardness. It is second only to diamond. There are three kinds- ruby, sapphire, and emery. Ruby is red gemstone corundum, sapphire is blue gemstone corundum. Emery is an "impure" kind of corundum used to make abrasive powder. Sapphire is the third most precious gemstone,ruby being second, and diamond as first.