Site of the old Carvoeiro coal mine
Coal is a combustible sedimentary rock, black or brown in color, which occurs in strata called coal seams. The harder forms, such as anthracite, can be considered metamorphic rocks due to subsequent exposure to high temperature and pressure. It is basically composed of carbon, sulphur, hydrogen, oxygen and nitrogen, as well as trace elements[citation needed]. The higher the carbon content, the purer it is considered to be. There are four main types of coal: peat, lignite, hard coal and anthracite (in ascending order of carbon content). It is extracted from the ground by open-cast or underground mining.
Among the various fuels produced and preserved by nature in fossilized form, coal is believed to be the most abundant. Along with coke and coal tar, its by-products are vital to many modern industries.