Coal is a combustible black or brownish-black sedimentary rock that is in layers, called coal beds or coal seams. The term “coal” is to describe a variety of different materials, but no two coals are exactly alike. When matching specific types of coal to a particular application, it entails many different mechanical strengths, chemicals, and properties, such as heating values, ash melting temperatures, sulfur as well as other impurities must be considered. Coal has different physical and chemical characteristics depending on the locations it is found. Coal is used as the primary source of energy utilized in the world. Coal is also one of the largest amounts shipped by ocean and land worldwide. Metallurgical, Steam, and Blue Gem Coal is the most common coal mined in the U.S. as well as being sold throughout the world. Coal is classified into four general categories. They range from lignite through sub-bituminous and bituminous to anthracite. This is due to the progressive response of individual deposits of coal to increasing heat and pressure. The carbon content of coal supplies most of its heating value, but other factors also influence the amounts of energy it contains per unit of weight. Much of the coal in this country falls in the bituminous and sub-bituminous categories. Bituminous coal is located throughout the Eastern and Mid-continent coal fields, while sub-bituminous coal is found in the Western states. Lignite ranks the lowest of the coals. Most lignite is mined in Texas, but large deposits also are found in some Gulf Coast states.