The natural materials in which the metals or their compounds are found in the earth are called minerals. Example: Aluminium occurs in the earth's crust in the form of two minerals, bauxite Al 2 O 3 . 2 H 2 O and clay Al 2 O 3 . 2 SiO 2 . 2 H 2 O .

Minerals from which metals are extracted profitably are called ores. Example: Aluminium occurs in the earth's crust in the form of two minerals, i.e., bauxite Al 2 O 3 . 2 H 2 O and clay Al 2 O 3 . 2 SiO 2 . 2 H 2 O . Out of these two minerals, aluminium is profitably extracted from bauxite but not from clay. Therefore, ore of aluminium is bauxite.

The unwanted impurities such as earthy, sandy and rocky materials associated with the ores are called gangue or matrix. Gangue particles are removed from the ore by different methods like gravity separation or hydraulic washing, froth floatation, electromagnetic separation, leaching.