Coal beds are composed of coal bands or layers sometimes separated by thin sedimentary rock layers (usually shale) called partings. Coal bands are described as types and lithotypes. Coal bands are formed from peat, which is mostly composed of plant debris with some amount of minerals. The thin sediment partings in most coals form when the original coal-forming peats are flooded, but at least one parting in Kentucky formed from volcanic ash that fell in the coal-forming peat.

Coal types and lithotypes are composed of altered plant parts such as roots, leaves, stems, and spores. Through the process of peatification and then coalification, distinctive plant parts partially decompose and are altered into tiny organic particles and gels called macerals. The coal-forming peat and subsequent coal also contain disseminated inorganic particles, called minerals. The proportion and types of macerals and minerals in a coal bed influence its quality and how it can be used.

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