Geology and Coastline
Jet is a specific type of fossilised wood, made from the branches and trunks of Jurassic trees which were washed down an ancient river system, flowing into the subtropical shallow sea.
When they reached the sea, the branches and trees became waterlogged and sank, and were quickly covered with silt and fallen debris from the rich aquatic life above.
180 million years later these tree remains are still there, compressed and fossilised by hundreds of feet of overlying rock.
Where does jet come from?
In geological terms, jet is found in the Upper Lias rocks from the Lower Jurassic era. The best jet comes from a band of hard shale about 3 metres thick, known as the jet rock. This layer can be seen rising and falling in the cliff face from Boulby Cliffs in the north, to Robin Hoods Bay in the south.
What is the difference between coal and jet?
Coal was formed in the Carboniferous era, around 300 million years ago. It is the fossilised remains of a mass of vegetation, rather than individual tree trunks. As coal is made under different conditions it lacks the hardness and structure of jet and cannot be cut and polished the same way.
Where can I find jet?
Jet can still be found in the cliffs, to the north and south of Whitby, but in many places it is too high up, or the danger from rock falls is too great even for the experienced collector.
These days jet is more commonly collected on the beach. As the cliffs erode, new seams become exposed and pieces of jet fall into the sea. As jet is very light these pieces are eventually found washed up on the shoreline, particularly after winter storms.
How can I identify jet?
Jet feels light and warm to the touch. If it is drawn across a piece of dry sandstone, or fine sandpaper, it leaves a brown, not black, streak. If the streak is black it is coal. Generally the lighter colour the streak , the higher quality the jet.
How can I be sure of the quality of jet?
Jet is found in a number of places around the world, but the special conditions in which jet became fossilised are uncommon, and good quality jet is very rare. The conditions in Whitby’s jet rock were perfect so Whitby jet is widely considered to be the best in the world. However, not all Whitby jet is of high enough quality for use in jewellery.
Robinson’s Jet takes pride in using only the best quality Whitby jet. You can be sure that any jet jewellery purchased in our shop will be of the highest quality.