Coal is an organic clastic sedimentary rock composed mostly of ancient plant material.
Pictured above, from left to right: Anthracite coal (the highest grade), Lignite or brown coal (the lowest grade), Bituminous coal (an intermediate grade, and most commonly used)
Coal is a combustible black or brown rock containing carbonaceous material that burns. It consists of carbonized ancient plant matter that is solid, but brittle. Originally deposited as leaf litter and plant remains in a fresh-water environment, the material first begins as peat, as in a peat bog. Over time it is compressed, dried, and modified by heat and pressure by sedimentary layers that cover it and by the proximity to the heat of the earth.This process is called 'coalification'. The peat then undergoes diagenesis with depth of burial and tectonic forces. Coal results as the end product of this diagenetic process. The more complete the process, the higher the grade, or rank of the coal.
Texturally, coal is subdivided into four main classes: lignite, subbituminous, bituminous, and anthracite. This last class is the hardest coal and contains the most carbon. Lignite is the least dense coal with lower carbon content. This sedimentary rock is composed mostly of carbon, hydrogen, oxygen, nitrogen (volatile hydrocarbons), and lesser amounts of ash, sulfur, and trace elements.
In Colorado, anthracite is found in Gunnison and Pitkin counties, and lignite is found in Adams and Elbert counties. Bituminous and subbituminous coals make up the majority of Colorado's coal resources, and are mined as clean low-sulfur but high-heat content coal products. Most of the electricity generated in Colorado comes from coal that is mined within the state. Coal is prolific in over 28 percent of Colorado's surface area. Most of the mineable coal resources today are located on the western slope.