noun. An extremely hard mineral, aluminum oxide, Al₂O₃, sometimes containing iron, magnesia, or silica, that occurs in gem varieties such as ruby and sapphire and in a common black, brown, or blue form used chiefly in abrasives.

noun. Alumina, or the oxid of the metal aluminium, as found native in a crystalline state.

noun. 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.

noun. An extremely hard mineral, a form of aluminum oxide with the chemical formula Al2O3, that occurs in the form of the gemstones sapphire and ruby; it is used as an abrasive.

noun. very hard mineral used as an abrasive

"Aluminum oxide, when it occurs in crystals rather than on pans, makes up the abrasive called corundum, and is also the principal material of rubies and sapphires the gem colors come from chromium and titanium impurities."

mineral  

adamantine spar  alumina  aluminium oxide  aluminum oxide  oriental amethyst  

Carborundum  

aberdonian  bauxite  beryl  bone dust  carnelian