Coal is a combustible, carbon-rich sedimentary rock formed from the accumulation and subsequent transformation of ancient plant matter through processes of burial, heat, and pressure over vast geological timescales. This process, known as coalification, gradually concentrates the carbon content while reducing the volatile organic compounds. The quality and physical properties of the rock vary significantly, resulting in different ranks of coal, such as lignite, sub-bituminous, bituminous, and anthracite, which are differentiated by their relative carbon content and metamorphic degree. Historically and currently, it serves as a major global energy source, primarily used in power generation to produce electricity by burning the material to create steam and drive turbines. Its composition is predominantly carbon, alongside trace minerals that dictate its calorific value.