Jet is a compact form of lignite, a type of brown coal, which can be polished and used as a gemstone. It forms from the decomposition of wood under extreme pressure and is known for its deep black color. Jet has been used since prehistoric times for jewelry and ornamental objects and is significant in understanding the early use of coal and organic material in geology.
Reference: Scott, A. C., & Glasspool, I. J. (2007). “Observations and Experiments on the Origin and Formation of Inertinite Group Macerals.” International Journal of Coal Geology, 70(1-3), 53-66.