coal

Black or blackish mineral substance formed from the compaction of ancient plant matter in tropical swamp conditions. It is used as a fuel and in the chemical industry. Coal is classified according to the proportion of carbon it contains. The main types are anthracite (shiny, with about 90% carbon), bituminous coal (shiny and dull patches, about 75% carbon), and lignite (woody, grading into peat, about 50% carbon). Coal can be burned to produce heat energy, for example in power stations to produce electricity. Coal burning is one of the main causes of acid rain, which damages buildings and can be detrimental to aquatic and plant life.