As geological processes apply pressure to dead biotic material over time, under suitable conditions, its metamorphic grade increases successively into:

Peat, considered to be a precursor of coal, has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water. It is also used as a conditioner for soil to make it more able to retain and slowly release water.
Lignite, or brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation. Jet, a compact form of lignite, is sometimes polished and has been used as an ornamental stone since the Upper Palaeolithic.
Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal, is used primarily as fuel for steam-electric power generation and is an important source of light aromatic hydrocarbons for thechemical synthesis industry.
Bituminous coal is a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of bright and dull material; it is used primarily as fuel in steam-electric power generation, with substantial quantities used for heat and power applications in manufacturing and to make coke.
\"Steam coal\" is a grade between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use, it is sometimes known as \"sea coal\" in the US. Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating.
Anthracite, the highest rank of coal, is a harder, glossy black coal used primarily for residential and commercial space heating. It may be divided further into metamorphically altered bituminous coal and \"petrified oil\", as from the deposits in Pennsylvania.
Graphite is one of the more difficult coals to ignite and is not commonly used as fuel — it is mostly used in pencils, and when powdered, as a lubricant.

Rhondda: Meaningful Nature