What type of rock is coal?

Coal is a sedimentary rock. It forms from accumulated plant material—especially in ancient swamp environments—that was buried, compacted, and chemically altered over millions of years.

The process begins when dead vegetation becomes peat in a waterlogged setting, where limited oxygen slows decomposition. As more sediment accumulates above it, pressure and heat drive out moisture and volatile compounds while increasing the carbon concentration. This gradual transformation is called coalification.

Coal is unusual among familiar sedimentary rocks because its material is mainly organic rather than mineral grains. Its rank reflects the degree of alteration: lignite is relatively young and low in carbon, bituminous coal is more mature, and anthracite is the hardest, highest-carbon variety.

Coal is sometimes confused with a metamorphic rock because heat and pressure are involved in its formation. However, ordinary coal is classified as sedimentary because it originates from deposited plant remains and forms in layers. Extreme metamorphism can alter coal further, but that does not change the standard classification used in introductory geology.