At various times in the geologic past, the Earth has had dense forests in low-lying wetland areas. Due to natural processes such as flooding, these forests were buried underneath soil. As more and more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. As the process continued the plant matter was protected from biodegradation and oxidation, usually by mud or acidic water. This trapped the carbon in immense peat bogs that were eventually covered and deeply buried by sediments. The high pressure and high temperature over millions of years caused loss of water, methane and carbon dioxide and an increase in the proportion of carbon and eventually dead vegetation was slowly converted to coal. This conversion of dead vegetation into coal is called coalification. Thus, first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed.

Lignite: Lignite coal, aka brown coal, is the lowest grade coal with the least concentration of carbon.

Subbituminous: Subbituminous coal is black in colour and dull (not shiny), and has a higher heating value than lignite.

Bituminous: Bituminous coal is a middle rank coal between subbituminous and anthracite. Bituminous usually has a high heating (Btu) value and is the most common type of coal used in electricity generation in the United States. Bituminous coal appears shiny and smooth when you first see it, but look closer and you may see it has layers.

Anthracite: The highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter.

China is the top producer with 3523 million tonnes in 2017 and 46% of share in total world production of Coal.

India is a distant second producer with 716 million tonnes in 2017 and just 9% of share in total world production of Coal.

United States, Australia, Indonesia and Russia are other major producers of Coal in the world.

China consumed nearly 4650 million tonnes of Coal in 2017 amounting to 51% of total world consumption.

India is a distant second consuming just about 1000 million tonnes in 2017 amounting to 12% of total world consumption with USA coming third.

Of all the fossil fuels, coal has the most widely distributed resources. Coal is mined on all continents except Antarctica. However, many such resources have no economic value (much value has been destroyed by shale gas fracking). The largest resources are found in the United States, Russia, China, Australia and India with the table below showing proven reserves: