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.
The matte black variety is the most common, and is an often overlooked stone, being useful for protection whilst journeying or in grounding and protecting the physical body during out-of-body experiences. In Victorian times it was often used in mourning pieces:, and so is useful for absorbing and transforming negativity and releasing stress - especially in the grieving process.
There is also a variety of jet that takes a high polish, which results in a metallic / bronze finish: this comes from Africa, and from Russia, and is very powerful - holding a piece at your solar plexus creates a sphere of energy that will surround and enclose the body and aura, enabling you to experience one-ness and Unity of the Self. NB: this type may fracture if dropped, and can sometimes lose its form if exposed to excess moisture, as it formed within the ground, rather than on the seabed.