What is also called black gold?

Known as coal seams, coal is a combustible sedimentary rock that is black or brownish-black in colour. With varying proportions of additional elements, primarily hydrogen, sulphur, oxygen, and nitrogen, coal is mostly composed of carbon. Dead plant matter decomposes into peat, which is then transformed into coal by heat and pressure from deep burial over millions of years. The vast majority of the world's tropical land areas were covered by coal forests, or former wetlands, during the late Carboniferous (Pennsylvanian) and Permian periods. Many large coal deposits, meanwhile, come from the Mesozoic and Cenozoic eras, which are much younger than this.