Jet is a black or dark brown variety of coal called lignite. It is a combination of carbon plus hydrocarbon compounds. The name is believed to be derived from the name of the town and river Gagas in ancient Turkey, where it or a similar material was found.

Jet was formed by the lithification of submerged driftwood in sea-floor mud.

The northeast coast of England has long been the best source of the worlds finest Jet, but it can also be found in the United States, Poland, France, Germany, Spain, India and Russia.