Emery is a very hard rock type used to make abrasive powder. It largely consists of the mineral corundum (aluminum oxide), Industrial emery may contain a variety of other minerals and synthetic compounds such as magnesia, mullite, and silica.

It is black or dark gray in color, less dense than translucent-brown corundum with a specific gravity of between 3.5 and 3.8. Because it can be a mix 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 others such as magnetite is near 6.

Crushed or naturally eroded emery (known as black sand) is used as an abrasive — for example, on an emery paper & board , as a traction enhancer in asphalt and tarmac mixtures, or as used in mechanical engineering as Emery cloth.