Coal is a combustible sedimentary rock formed from ancient vegetation which has been consolidated between other rock strata and transformed by the combined effects of microbial action, pressure and heat over a considerable time period. This process is commonly called ‘coalification’. Coal occurs as layers or seams, ranging in thickness from millimetres to many tens of metres. It is composed mostly of carbon (50–98 per cent), hydrogen (3–13 per cent) and oxygen, and smaller amounts of nitrogen, sulphur and other elements. It also contains water and particles of other inorganic matter. When burnt, coal releases energy as heat which has a variety of uses. Coal is broadly separated into brown and black which have different thermal properties and uses.

Brown coal (lignite) has a low energy and high ash content. Brown coal is unsuitable for export and is used to generate electricity in power stations located at or near the mine.

Black coal is harder than brown coal and has a higher energy content. In Australia anthracite, bituminous and sub-bituminous coals are called black coal whereas in Europe, sub-bituminous coal is referred to as brown coal.

Thermal (steaming) coal is black coal that is used mainly for generating electricity in power stations where it is pulverised and burnt to heat steam generating boilers.

Metallurgical (coking) coal is black coal that is suitable for making coke, which is used in the production of pig iron. These coals must also have low sulphur and phosphorus contents, and are relatively scarce and attract a higher price than thermal coals.

Coke is a porous solid composed mainly of carbon and ash and is used in blast furnaces that produce iron.

Coal reserves are discovered through exploration. Modern coal exploration typically involves extensive use of geophysical surveys, including 3D seismic surveys aimed at providing detailed information on the structures with the potential to affect longwall operations, and drilling to determine coal quality and thickness.