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Coal is an important natural resource predominantly used for energy production. It is a combustible, sedimentary rock primarily made of carbon, hydrogen, sulfur, oxygen, and nitrogen. It has been a source of energy for various industries and homes for many centuries.

Coal forms over millions of years through the accumulation and transformation of organic plant material due to heat and pressure. The formation process involves several stages: 1. Plant material (such as fallen leaves, branches, and tree trunks) accumulates in swamps or peat bogs, where the low-oxygen environment slows down decomposition. 2. The dead plant matter on the surface turns into peat, a soft, brown material rich in organic matter. 3. Over time, the peat layer becomes buried under layers of sediment, including sand, silt, and clay. As the depth increases, so does the pressure and temperature. 4. Heat and pressure cause the peat to undergo chemical and structural changes, altering its composition. 5. Over millions of years, the peat transforms into lignite, a low-quality, brownish-black coal known for its high moisture content. 6. As the process continues, lignite turns into sub-bituminous coal, bituminous coal, and eventually anthracite, a high-quality, black, shiny coal with the highest carbon content.

Coal belongs to the sedimentary class of rocks. Sedimentary rocks are formed from the accumulation and cementation of mineral and organic particles. They are typically deposited in layers called strata and often include fossils. Other examples of sedimentary rocks include limestone, sandstone, and shale. In summary, coal is a sedimentary rock that forms from the accumulation and transformation of organic plant material under heat and pressure over millions of years. Its formation process involves several stages, including peat formation, burial by sediment, and transformation into various coal types.