Coal is a combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Coal is composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, with smaller quantities of sulfur, oxygen and nitrogen.

Coal begins as layers of plant matter accumulating at the bottom of a body of water. For the process to 

continue, the plant matter must be protected from biodegradation and oxidization, usually by mud or acidic water. This traps their carbon in immense peat bogs that are eventually covered over and deeply buried by sediments. Under this compression the plant material is metamorphosed into coal: over time, the chemical and physical properties of the plant remains are changed by geological action to create a solid material.

The wide shallow seas of the Carboniferous period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the Coal gap in the Lower Triassic, where coal is rare: presumably a result of the mass extinction which prefaced this era. Coal is even known from Precambrian strata, which predate land plants: this coal is presumed to have originated from algal residues.

Coal, a fossil fuel, is the largest source of energy for the generation of electricity worldwide, as well as one of the largest worldwide anthropogenic sources of carbon dioxide releases. Gross carbon dioxide emissions from coal usage are slightly more than those from petroleum and about double the amount from natural gas. 

Coal is extracted from the ground by mining, either underground by shaft mining through the seams or in open pits.

There is even a type of coal known as Jet that is hard enough to be used in jewelery.

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