| description | Coal is a combustible black or brownish-black sedimentary rock composed primarily of carbon along with variable quantities of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen, derived from fossilized organic remains. Coal is subdivided in terms of rank. The hardest form or highest rank is anthracite coal, which can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Intermediate rank coal is bituminous and low rank coal includes lignite. http://en.wikipedia.org/wiki/Coal |