Jet
The much-prized mineral known as jet, commonly associated with Whitby, is present across much of the Tees Valley and North Yorkshire. The name jet comes from the Greek term for jet ‘Lithos Gagates’ which means stone of Gagas (Gagas is a town in Turkey). In French this translates into gaiet or jaiet and then into English as jet.
The earliest examples of worked jet date from around 10,000BC. Since the Bronze Age, people in Yorkshire have seen jet as a valuable commodity, often recovered from burial mounds (barrows) in the form of beads, bracelets and other precious items. Jet became universally popular when in 1861, Albert, Prince Consort to Queen Victoria died. Queen Victoria entered a long period of mourning declaring only jet jewellery was to be worn in court. The jet industry responded and by 1872, it is estimated that there were around 200 workshops, with over 1400 people employed to turn the raw material into beautiful jewellery. This is compared to 2 jet workshops employing 25 people in 1832.
Jet formed from the fossilised trunks of Araucaria trees - similar to the modern day Monkey Puzzle. The trees grew on a Jurassic landmass away to the north west, a little over 180 million years ago. When they died and fell in the river they were carried to the ancient sea. The trunks would eventually become waterlogged and sink to an oxygen-poor sea floor where decomposition was either slow or non-existent. Gradually, the sunken logs became buried beneath more sediment until their once rounded trunks were flattened into thin planks by the weight. The rock member in which jet resides belongs to the Whitby Mudstone Formation and is known as the Jet Rock. The mineral comes in two forms known as hard or soft jet, with the former most prized for working and found below a thin limestone known as the Top Jet Dogger. Soft jet can be found slightly above the Top Jet Dogger as well as in the deltaic deposits of the Middle Jurassic. Native jet often preserves the shape and texture of the tree that made it and this can sometimes be seen in the finished jewellery.
When it is found, it is a dull grey brown and unspectacular to look at, but it is easily worked and after final polishing jet takes on a deep lustrous shine.
In the early days of exploitation, jet was simply picked off the beaches, but as demand grew mining became necessary. Dessing is an early method of working jet. It involved a man being lowered over the cliff side where he would break off pieces of jet. As demand grew, safer methods were sought to get the jet and mining started. Whitby was the main manufacturing centre but much of the jet was obtained from small mines located along our part of the coast and inland, for example in the hills around Guisborough and Great Ayton. Jet was usually mined by driving tunnels, known as adits, into the cliffs or hillsides, several metres below the Top Jet Dogger. The miners would then work into the roof, standing on the spoil they produced to reach the ceiling as the roof gained in height, until they met the Top Jet Dogger which they used as a strong roof for their workings. The miners sold the jet they collected to a jet merchant who would then sell it on to a jet workshop where they would turn it in to jewellery.
How to identify jet
- Look for evidence that jet was once a tree - sometimes growth rings can be seen.
- Jet will leave a brown mark when scratched on a piece of unglazed white pottery.
- If you burn jet it will smell like burning coal.
- Jet is warm to the touch.
- Jet will never fade in sunlight.