Coal is a mineral found in the earth. It is organic, hence it is mainly composed of carbon, hydrogen and oxygen, with trace amounts of other elements such as nitrogen and sulphur. It also contains a varying amount of water, as well as some inorganic substances, most of which make up the ash when combusted. Coal is formed via a natural process, where organic matter (from millions of years old plant matter) decay and are subject to heat and pressure. The varying amounts of heat and pressure give rise to the different types of coal that is produced. This process is known as coalification. The variety in the formation process causes two different types of coal to be formed- black and brown coal. Black coal is softer, and has more energy and lower water content than the brown variant. Brown coal is harder, and burns with about a quarter of the amount of energy as black coal. It also has higher carbon and lower moisture compared to black coal.