Minerals are naturally occurring chemical compounds that form in pure crystalline structures within the Earth. They originate as igneous rocks cool and crystallize below the Earth's surface. Minerals have distinct chemical compositions and properties including color, streak, luster, hardness, cleavage, diaphaneity, and magnetism. These properties are determined by a mineral's composition and crystalline structure. Color, for example, is usually caused by electromagnetic radiation interacting with a mineral's electrons. Hardness refers to a mineral's resistance to scratching and is measured using the Mohs scale. Cleavage describes a mineral's tendency to break along planes of weaker atomic bonding.