emery

emery, granular rock consisting of a mixture of the mineral corundum (aluminum oxide, Al₂O₃) and iron oxides such as magnetite (Fe₃O₄) or hematite (Fe₂O₃). Long used as an abrasive or polishing material, it is a dark-coloured, dense substance, having much the appearance of an iron ore. In addition to corundum and iron oxide, emery sometimes contains diaspore, gibbsite, margarite, chloritoid, and sillimanite. Emery has been worked from very early times on the Greek island of...