a common mineral, aluminium oxide, Al2O3, notable for its hardness (9 on the Mohs scale). Transparent varieties, including the ruby and sapphire, are prized gems; translucent varieties are used as abrasives.
a common mineral, aluminium oxide, Al2O3, notable for its hardness (9 on the Mohs scale). Transparent varieties, including the ruby and sapphire, are prized gems; translucent varieties are used as abrasives.