Coal is a combustible black or brownish black sedimentary rock. Such sedimentary rocks usually occur in rock formations or veins called coal beds or coal seams. Because of later exposure to elevated temperature and pressure, harder forms of coal can be considered metamorphic rocks, such as anthracite. Coal is mainly composed of carbon, together with different amounts of other elements, mainly hydrogen, sulfur, oxygen and nitrogen. Historically, coal has been used as an energy resource, mainly for combustion to produce electricity heat, and can also be used for industrial purposes, such as refining metals, or producing chemical fertilizers and many chemical products. As a fossil fuel, coal was formed when ancient plants were buried underground before being decomposed and decomposed, converted into peat, then converted into lignite, then sub-bituminous, then bituminous, and finally anthracite. The hydrocarbons produced by coal act under the pressure and temperature conditions of crustal moving air, and the resulting carbonized fossil minerals, that is, coal is plant fossils. This involves a long period of biological and geological processes.