A black or brownish-black combustible sedimentary rock, formed from subjecting the dead plant matter in peat to pressure and heat for millions of years. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. It is typically mined to be used as a cheap, albeit polluting, source of energy or for the production of steel. Coal is associated as a punishment "gift" for naughty children during Christmas.