Minerals are naturally occurring substances that make up rocks and have distinct internal structures. Over 2000 minerals have been identified, though only a few make up most rocks. The physical and chemical conditions during formation determine a mineral's properties like color, hardness, crystal structure, luster and density. Minerals are classified as metallic, which include ferrous, non-ferrous and precious metals, or non-metallic like mica, salt and limestone. Energy minerals used as fuels include coal, petroleum and natural gas.