corundum

[kuh-ruhn-duhm]
noun
a common mineral, aluminum oxide, Al 2 O 3 , notable for its hardness: transparent varieties, as sapphire and ruby, are used as gems, other varieties as abrasives: often made synthetically.

Origin:
1720–30; < Tamil kuruntam; akin to Sanskrit kuruvinda ruby

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Collins
World English Dictionary
corundum (kəˈrʌndəm)
 
n
a white, grey, blue, green, red, yellow, or brown mineral, found in metamorphosed shales and limestones, in veins, and in some igneous rocks. It is used as an abrasive and as gemstone; the red variety is ruby, the blue is sapphire. Composition: aluminium oxide. Formula: Al2O3. Crystal structure: hexagonal (rhombohedral)
 
[C18: from Tamil kuruntam; related to Sanskrit kuruvinda ruby]

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Etymonline
Word Origin & History

corundum
1728, from Anglo-Ind., from Tamil kurundam "ruby sapphire" (Skt. kuruvinda).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
corundum   (kə-rŭn'dəm)  Pronunciation Key 
An extremely hard mineral occurring in many colors, either as shapeless grains or as rhombohedral crystals. It also occurs in gem varieties such as ruby and sapphire and in a dark-colored variety that is used for polishing and scraping. Corundum is found in igneous and carbonate rocks. Chemical formula: Al2O3.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Rubies, valued as precious gems, are the mineral corundum in its red form.
Rubies are formed of a mineral called corundum, comprised of aluminum oxide.
No doubt large corundum crystals and pieces of corundum are found each year.
The cordierite occurs along with sillimanite, corundum, and kyanite.
Images for corundum
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