The mineral alumina (Al2O3), as found native in a crystalline state. Transparent varieties are used as gemstones, including sapphire, which is the fine blue variety; the oriental ruby, or red sapphire; the oriental amethyst, or purple sapphire; and adamantine spar, the hair-brown variety. It is the hardest substance found native, next to the diamond.
A gem species that is found in many colors. Red corundum is called ruby and all other varieties are called sapphire. Blue Sapphire is best known, however pink, green, and yellow sapphire are also relatively popular.