A natural mixture of corundum with magnetite or with hematite and spinel. Emery has been used for centuries as an abrasive or polishing material. Because the mixture is very intimate and appears to be quite homogeneous, it was considered to be a single mineral species until the middle of the nineteenth century. The aggregate has a gray-to-black colour and is extremely tough and difficult to break. The specific gravity varies from 3.7 to 4.3, depending upon the relative amounts of the constituent minerals. The hardness is about 8 (Mohs scale), less than that of pure corundum which is 9, and is more dependent upon the physical state of aggregation than on the percentage of corundum.