Sapphire is a variation of corundum. The chemical formula of sapphire is Al<sub>2</sub>O<sub>3</sub>, which means that this stone is practically aluminum oxide, i.e., alumina. Since almost everybody has seen aluminum metal and knows how it looks, it is really amazing that this gem can be so magnificently different. The hardness of sapphire stones is amongst the hardest in the mineral world, approaching 9 out of the possible 10 on the Mohs scale.