What is Jet?
What is Jet?
In the Journal of Gemmology XVII No.1 (1980) H.Muller presents the following analysis.
Refractive index 1.64-1.68 Specific gravity 1.3 -1.4 Mohs scale 2.5 - 4.0
Carbon 75.2% Hydrogen 7.0% Nitrogen 0.7% Sulphur 4.6% Oxygen 12.5%
Also contained are trace elements of silicon, potassium, calcium, iron, copper, and aluminium.

Jet is a black organic rock that forms when pieces of woody material are buried in sediment and are coalified. Though very similar to coal, it is less friable. Jet can be cut, carved, and polished to a bright luster. People have used jet for thousands of years to produce gemstones, beads, and many other objects. Jet is one of just a few organic gemstones. It is the material that inspired the phrase "jet black," which means "as black as possible."

How Does Jet Form?
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.

Physical Properties of Jet
Jet has a few properties that make it useful and desirable. These have dictated its use across the centuries. The first of these properties is its ability to be easily carved or cut into shapes. Jet is soft and has a uniform texture, which allows it to be carved with precision.
Jet can be rubbed to a nice matte finish or polished to a very bright luster. Done well, these finishes enhance the aesthetic value of carved or cut pieces of jet.
Jet also has a low specific gravity. A strand of jet beads or a large cabochon thus weighs about 50% less than if they were made from agate, jasper, quartz, or other mineral material. This allows the beads to be worn with greater comfort and the brooch to be worn without placing as much stress or hanging awkwardly on the garment.