|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]|
|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.