Coal is a fossil fuel that formed during the Carboniferous period when plant materials were buried underwater, preventing their decomposition. Over time, due to pressure and heat, various grades of coal may be formed. For the generation of electricity, this fossil fuel is very important. Lignite, or brown coal is used for electric generation in power plants. Bituminous coal which is a little harder is used for steam-electric power generation. Anthracite, which is considered to be a metamorphic rock due to the extensive heat and pressure applied to this material for it to form, is hard coal, which has a shiny appearance. Anthracite is used in home and commercial space heating. Although this type has the most energy stored, it is harder to ignite, but, it burns cleaner and with fewer pollutants.