JET

Jet (Lignite) 

Jet is produced by high-pressure decomposition of wood and is found in two forms: hard and soft, with hard lignite occurring with carbon compression and salt water, and soft lignite resulting from carbon compression with fresh water. Jet is the lowest rank of coal, and is a gemstone. Unlike many gemstones, Jet is not a mineral, but is rather a mineraloid. It is derived from wood that has changed under extreme pressure.