Webster's English Dictionary

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corundum

co.run.dum \k*-'r*n-d*m\ n [Tamil kuruntam, fr. Skt kuruvinda ruby] : a 
   very hard mineral Al2O3 that consists of aluminum oxide oc curring massive 
   and as variously colored crystals which include the ruby and sapphire, that 
   can be synthesized, and that is used as an abrasive (hardness 9, sp. gr. 
   3.95-4.10)