1. n. [Geology]
A carbon-rich sedimentary rock that forms from the remains of plants deposited as peat in swampy environments. Burial and increase in temperature bring about physical and chemical changes called coalification. Because of the organic origin of coal, it cannot be classified as a mineral. The main types of coal, anthracite, bituminous coal and lignite, can be distinguished by their hardness and energy content, which are affected by their organic content as well as their conditions of formation. Natural gas associated with coal, called coal gas or coalbed methane, can be produced economically from coal beds in some areas. In some basins coals form source rocks.