Definitions

from 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • 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.

from 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.

from The Century Dictionary and Cyclopedia

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. very hard mineral used as an abrasive

Etymologies

Tamil kuruntam.
(American Heritage® Dictionary of the English Language, Fourth Edition)
From Tamil குருந்தம் (kuruntam, "ruby"). (Wiktionary)

Examples

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  • a crystalline form of aluminium oxide and one of the rock-forming minerals. It is naturally clear, but can have different colors when impurities are present.

    July 18, 2008