Jet is not considered a true mineral, but is classed as a mineraloid as it has organic origin, being derived from wood under extreme pressure.
Coniferous trees from the Jurassic and Cretaceous periods are mostly the type of tree which has formed Jet, which is also called Lignite and may have inclusions of pyrite (Fools Gold - an iron sulphide) . The English adjective "Jet-Black" is derived from this material as it is normally so black. Jet found in the north of England is approximately 180 million years old and is derived from wood similar to the "monkey puzzle tree".
Jet is a softer gem having an Mohs hardness ranging between 2.5 and 4, thus being relatively easy to carve.
One way to distinguish jet from other imitations is to rub jet against unglazed porcelain where authentic Jet will leave a chocolate brown streak.
Some examples of Jet are shown below.