Lignite
Lignite, often called brown coal, is a soft sedimentary rock. It is brown in color and combustible. The rock is created from naturally compressed peat. It has a carbon content around 25–35%. It is thought to be the lowest rank of coal. This is because of its low heat content. Lignite has a  high amount of moisture when it is taken from the ground. This can explain its low carbon content. The rock is mined all around the world. It is mostly used as a fuel for steam-electric power generation.