Emery (or corundite) is a dark granular rock used to make abrasive powder. Emery is named after Cape Emeri on the Island of Naxos where it was first discovered. It largely consists of corundum mixed with other species such as the iron-bearing spinels, hercynite, and magnetite, and rutile. Industrial emery may contain a variety of other minerals and synthetic compounds such as magnesia, mullite, and silica. It is black or dark grey in colour, less dense than translucent-brown corundum with a specific gravity between 3.5 and 3.8. Because it can be a mixture of minerals, no definite Mohs hardness can be assigned: the hardness of corundum is 9 and that of some spinel-group minerals is near 8, but the hardness of other components such as magnetite is near 6. Crushed or naturally eroded emery (known as black sand) is primarily used as an abrasive, for example, on emery board and emery cloth, and as a traction enhancing additive to road asphalt and tarmac. Trimmed blocks of quarried emery have also been used as architecural and structural stone due to their resistance to abrasion and weathering.