| n. | 1. | (Min.) 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. |