Coal is a sedimentary rock formed mainly from compacted and cemented plant remains.
Most coal began as vegetation accumulating in waterlogged swamps, where low oxygen slowed complete decay. Burial beneath later sediments increased pressure and temperature, gradually transforming peat into coal through a process called coalification. The main ranks are lignite, sub-bituminous coal, bituminous coal and anthracite, in increasing maturity.
Coal is chiefly made of carbon along with hydrogen, oxygen, nitrogen, sulfur and mineral matter. Its composition varies because the original plants, burial conditions and geological history vary. Coal seams are therefore not chemically identical, even when they occur in the same basin.
Coal is sometimes confused with charcoal. Charcoal is produced artificially by heating wood with limited oxygen, whereas coal is a natural geological material formed over millions of years. Coal has been important as a fuel and as a source of coke for ironmaking, but burning it releases carbon dioxide and other pollutants.