Jet from the best-known source, Whitby in North Yorkshire, UK, formed over 180 million years of high-temperature compression under the sea. Jet is a coherent form of carbon with a hardness of 4 on the Mohs scale – hard enough to take a good polish, which makes it a great choice for jewellery. Its technical name is lignite.
The name ‘jet’ comes from the French word jais, which is derived from the term for the ancient town of Gagae in southern Turkey. This region was known for its production of jet in Greek and Roman times.
Jet has been used since before the Bronze Age to make jewellery and functional items. Today, it is found in several places around the world – including Poland, Spain and the US – however Whitby has historically been the largest supplier of quality commercial quantities. The use of jet was promoted by locals in the early 1800s and then became very popular in the Victorian era.
This popularity was given a boost by Queen Victoria herself, who chose Whitby jet for her mourning jewellery after her husband Prince Albert passed away in 1861. Being affordable, jet then became popular for mourning jewellery among the public.
Due to being lightweight, one of its identifying features, jet can be made into quite large items – everything from large earrings to festooned necklaces.
After Queen Victoria died in 1901, the British were ready for a more colourful time and jet jewellery fell out of fashion.
Jet is still obtainable at Whitby, but where there once were 200 workshops carving jet at its peak of popularity, there are now less than five.
There are many imitations, as well as gems that can be mistaken for jet. It can be hard to identify so it is often best to use a process of elimination from substances of similar appearance.
Some common imitations include:
Black glass – Commonly called ‘French jet’, it is much heavier than the real thing, feels colder to the touch and includes air bubbles.
Gutta percha – A soft rubber compound invented in the mid-1800s, it is moulded then painted black to imitate jet. It is easily scratched and does not take a good polish.
Ebonite – This is a vulcanised rubber that is moulded and polished. It is harder than jet and will show less wear. It will also fade over time when exposed to sunlight.
Bog oak – Wood that has been buried in a peat bog and stained by the tannins to dark brown is called ‘bog oak’. It is carved but does not take as good a polish as jet, and the wood grain is visible. It comes from Ireland, so is often decorated with Irish themes.
Black onyx – A type of cryptocrystalline quartz, this mineral is much heavier and harder than jet.
Plastics – Plastic imitations of jet vary in their properties, but they are usually heavier and show evidence of moulding.
Another method for identifying jet is a streak test. If rubbed on a rough surface, jet shows a streak of dark brown colour. Onyx, glass and plastics will not produce a streak.
There is also a destructive test that uses a hot needle to burn the sample, which, if it is jet, will produce a characteristic smell. However, destructive tests are to be avoided and must only done by experienced gemmologists.