Emery (also known as iron spinel and hercynite) is a common impure variety of the mineral corundum.

It is black or dark gray in colour, less dense than translucent-brown corundum with a specific gravity of between 3.5 and 3.8 and does not form any regular large-scale crystalline structures. It has a Mohs hardness of 8.

In addition to aluminium oxide, emery contains an iron-bearing mineral such as magnetite or hematite and trace impurities such as magnesia, mullite , titania or silica.

Crushed or naturally eroded emery (known as black sand) is used as an abrasive.