co·run·dum   (kə-rŭn'dəm)    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. [Tamil kuruntam.]

Corundum

Co*run"dum\ (k?-r?n"d?m), n.; pl. Corundums (-d?mz). [Also corindon.] [From Hind. kurand corundum stone.] (Min.) The earth alumina, as found native in a crystalline state, 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.

Note: The name corundum is sometimes restricted to the non-transparent or coarser kinds. Emery is a dark-colored granular variety, usually admixed with magnetic iron ore.

corundum 

Main Entry: co·run·dum
Pronunciation: k&-'r&n-d&m
Function: noun
: a very hard mineral Al₂O₃ that consists ofalumina occurring in massive form and as variously colored crystals which include the ruby and sapphire

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.