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.

• 刚玉一种极为坚硬的矿物，氧化铝，Al2O3，有时含铁、氧化镁或硅土，存在于各种宝石中，如红宝石、蓝宝石，并且在磨蚀剂中以普通的黑色、棕色或蓝色形式出现