The American Heritage® Dictionary of the English Language, 4th Edition
n. 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.
n. 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.
the GNU version of the Collaborative International Dictionary of English
n. 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.
The Century Dictionary and Cyclopedia
n. Alumina, or the oxid of the metal aluminium, as found native in a crystalline state.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. very hard mineral used as an abrasive
Word Usage
"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."