Coal is not a mineral because it is organic, and minerals are inorganic. Minerals have a repeating crystalline structure and a homogeneous chemical profile. Coal has neither. It forms over millions of years from the compressed, heated remains of dead plants and animals.
According to the World Coal Organization, the chemical composition of coal includes carbon, sulfur, hydrogen, oxygen and nitrogen. The proportion of these and other elements in a coal vein depends on its location, age and the types of organic material that contributed to its formation.
The term "coal" refers to seven related substances. The deeper the coal is, the higher its quality. Peat is a popular fuel in certain European countries and has powerful absorptive ability. In that context peat is quite useful during oil spill cleanup. Lignite has a medium brown color and is only used to power electric generators. It is sometimes called "brown coal." Bituminous and sub-bituminous coals are used in steam power generators. Steam locomotives, however, are powered by steam coal, which is sometimes called "see-coal." Black, semi-shiny anthracite coal is the highest quality coal used for heat and power generation. The seventh coal type, graphite, is purer than anthracite but does not burn readily. It is used as pencil "lead" and as a powder lubricant.