Answer: corundum

AnswerCrossword Clue
CORUNDUMSapphires and rubies.
CORUNDUMRuby or sapphire
corunduma hard mineral
corundumcommon mineral, aluminum oxide, Al2O3, notable for its hardness: transparent varieties, as sapphire and ruby, are used as gems, other varieties as abrasives: often made synthetically