Coal

Coal is a combustible sedimentary rock formed from the compressed, buried remains of ancient plants. It is composed largely of carbon, along with hydrogen, oxygen, nitrogen, sulfur, and mineral impurities, and is the most abundant fossil fuel.

Coal is classified by 'rank', reflecting how much heat and pressure it has experienced. From lowest to highest rank these are peat (a precursor), lignite (brown coal), sub-bituminous, bituminous, and anthracite. Higher rank means more carbon, more energy per unit, and a harder, shinier rock.

For over two centuries coal powered the Industrial Revolution and electricity generation, though its combustion releases carbon dioxide and pollutants, making it central to climate and energy debates.

Coal forms from plant material that accumulates in waterlogged, oxygen-poor swamps and bogs, where it cannot fully decay and instead builds up as peat. Over millions of years, burial under sediment subjects the peat to increasing heat and pressure.

This 'coalification' progressively drives off water and volatile compounds and concentrates carbon, converting peat into lignite, then bituminous coal, and ultimately anthracite at the highest grades, which may verge on metamorphism.

Many of the world's great coal deposits formed during the aptly named Carboniferous Period, when vast tropical swamp forests covered the continents. Major coal regions include the Appalachian and Powder River basins (USA), China, India, Australia, Russia, and the coalfields of Europe.

Look for a relatively lightweight, black to brownish-black rock that is fairly soft (it can be scratched with a fingernail or knife depending on rank) and may leave a black mark on your hands. It is notably less dense than most rocks and is combustible.

Lignite is dull, brown-black, and may show woody texture; bituminous coal is black with dull and shiny bands; anthracite is hard, shiny, almost metallic, and breaks conchoidally. The streak is brown to black.

Look-alikes include jet (a hard, polishable coal variety used in jewelry), shale (heavier, not combustible, often grey), and obsidian (glassy, much harder, dense). Coal's low density, sooty feel, black color, and combustibility distinguish it.

Coal has been a dominant energy source for generating electricity and heat and was the fuel of the Industrial Revolution. Bituminous coal is converted into coke, essential for smelting iron in blast furnaces to make steel.

Coal is also a feedstock for the chemical industry, yielding tar, ammonia, and a range of derivatives, and historically lit homes and powered railways and ships. The hard, lustrous coal variety jet has been carved into mourning jewelry and ornaments since antiquity.

Today coal's use is declining in many regions because burning it emits carbon dioxide and pollutants such as sulfur and mercury, central concerns in climate change and air quality, though it remains a major global fuel.