Lignite is a low-grade, brownish-black coal that represents an intermediate stage between peat and bituminous coal. It has a relatively low carbon content and high moisture content, making it less efficient as a fuel. Lignite is significant in studying the formation of coal, paleoenvironments, and the carbon cycle.
Reference: Taylor, G. H., Teichmüller, M., Davis, A., Diessel, C. F. K., Littke, R., & Robert, P. (1998). “Organic Petrology: A New Handbook Incorporating Advances in the Understanding of Organic Matter and Fossil Fuels.” Gebrüder Borntraeger.