Is coal a sedimentary or metamorphic rock?
Because coal undergoes physical and chemical changes as a result of increased heat, there is sometimes a misconception that coal is a metamorphic rock. Coal is a sedimentary rock.
Coal is altered through biological and burial-thermal processes into different ranks. Many sedimentary rocks are also altered through burial-thermal processes (increasing cementation, etc.). Coal is not what most geologists would consider metamorphic rock (with the possible exception of meta-anthracite, which is transitional with metamorphic rocks). Confusion arises because high-rank coals are considered the product of processes termed "low-grade burial metamorphism." Low-grade burial metamorphism is still below the level of temperature and pressure which forms typical metamorphic rocks (marble, quartzite, slate, etc.).