The material known as “jet” is very similar to coal, but the way that it forms is different. Most coal seams form when a swamp containing abundant woody material is buried; that woody material is then compacted, undergoes organic degradation, and is heated. The result is a coal seam.

Jet does not form in a seam. Instead, it forms when an individual piece of woody material such as a tree branch is washed into a body of water, becomes waterlogged, sinks to the bottom and is covered by organic-rich sediment. It is then compacted, degraded, and heated in isolation.

This produces a material that is similar to coal; however, this material is much more influenced by the geochemical environment of the surrounding organic-rich shale. Jet is thought to absorb oils released by the decay of oil-rich organic debris in the surrounding rock, such as algae and plankton. As the jet is transformed into a rock, it does not develop a system of fractures, known as “cleat,” which develops in a coal seam. This gives the jet a more uniform texture and a toughness that contrasts with the friability of coal removed from a seam.

The woody origin of jet has been confirmed because, when examined under magnification, jet contains the preserved cellular structure of the original woody plants. Some specimens display obvious plant structures without magnification.

Jet with wood grain: A thin piece of jet from a mine near Whitby, England. The surface of this piece shows some of the structures of the plant from which it was formed. The specimen is about 3 centimeters across.