Jet is an amorphous mineral, with no specific crystalline structure. It is a form of coal (carbon) that formed from wood that had become submerged in mud on the seabed.
It is an often overlooked stone, being useful for protection whilst journeying or in grounding and protecting the physical body during out-of-body experiences. For those with shamanic interests, especially of the Celtic kind, this is an interesting stone to explore. In the Victorian times jet was often used in mourning pieces: it is useful for absorbing and transforming negativity and releasing stress - thus may have been (and be) helpful in the grieving process.
The matte black variety is the most common, but there is also an African variety that takes a high polish, which results in a metallic / bronze finish. This is a very powerful piece: holding a piece at ones centre enables one to ground a sphere of energy that will surround and enclose the body and aura, enabling one to experience one-ness and Unity of the Self.