Generally, any carbon-rich material formed by the compaction and alteration of plant matter. More specifically, a rock containing more than 50% carbonaceous material by weight, and more than 70% by volume. A number of coal “ranks” or classes are recognized, based on the amount of heating and compaction undergone by the rock. These are (from least altered to most altered): peat, lignite (brown coal), sub-bituminous, bituminous, sub-anthracite, and anthracite. Coals can also be classified by “grade”, based on the amount of impurities (such as sulfur) present. High rank coals are often used to produce coke for steel-making, while mid- to low rank coals are generally used in heating and power generation.