coal, Solid, usually black but sometimes brown, carbon-rich material that occurs in stratified sedimentary deposits. One of the most important fossil fuels, it is found in many parts of the world. Coal is formed by heat and pressure over millions of years on vegetation deposited in ancient shallow swamps (see peat). It varies in density, porosity, hardness, and reflectivity. The major types are lignite, subbituminous, bituminous, and anthracite. Coal has long been used as fuel, for power generation, for the production of coke, and as a source of various compounds used in synthesizing dyes, solvents, and drugs. The search for alternative energy sources has periodically revived interest in the conversion of coal into liquid fuels; technologies for coal liquefaction have been known since early in the 20th century.
