Coal

Coal is a combustible black or brownish-black sedimentary rock composed primarily of fossilized carbon along with variable quantities of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. It usually occurs on planets where biological and geological process allow the accumulation of dead organic matter in layers of rock strata.

Primitive cultures have used coal for the production of electricity and/or heat, and it has also been used for industrial purposes, such as refining metals.