Geology & Gemmology

 

Jet can be found in many different countries around the world including Germany, Australia, France and Poland. In North America jet is found in the states of Illinois, Utah and Colorado. Pueblo Indians used the material to make items of jewellery in conjunction with turquoise and shell, this style of Native American jewellery is still popular in the US today.

Various deposits have also been found in the Baltic. In Spain, good quality jet is found in the provinces of Aragon, Asturias and Galicia. While in Russia and China, there is very poor-quality material which arguably shouldn’t even be referred to as jet. However, the jet found from around the small seaside town of Whitby in North Yorkshire, England has always been considered the finest and most prized because of its unrivalled mirror like shine, intense black colour, and stable nature when being worked.

 

The North Yorkshire coast of England is famous for its impressive shale cliffs towering hundreds of feet above the North Sea. These cliffs are rich with fossils and the many different strata are clearly visible within the rocks. It was the condition in which Whitby’s jet was able to fossilise which sets it apart from the rest.

The feel of jet is nothing like one might imagine, for it is incredibly light in weight and warm to the touch. Although it is classed as an organic gemstone, it is a mineral with a chemical composition very similar to coal, a hydrocarbon, with a maceral content of vitrinite, and not technically a stone at all.

 

There has been much discussion as to the origins of jet over the years. From the theory that jet is derived from a particular species of tree, to any kind of tree, to not being ligneous at all, but instead being an aggregate of bituminous matter. The latter was disproved in 1904 by A.C Steward who produced a series of photomicrographs of jet clearly showing its ligneous nature by revealing the presence of tracheid’s. In the 1980’s, much research was carried out to prove that jet was the fossilised remains of Araucaria Araucana, a distant relative of today’s Monkey Puzzle tree and the most prolifically growing species of tree throughout the Jurassic period, whereas today, research suggests that jet may originate from any kind of wood.

Whatever its species, the formation of jet began with trees falling into rivers and bogs and slowly making their way towards the sea, usually breaking up along the way. After becoming waterlogged, the wood would sink to the seabed where over time layers of sediment, detritus, shells and other marine life would bury it, starving it of oxygen and compressing the wood under thousands of tons of rock. It would take between 180 and 200 million years for the transition to take place from wood into hard, workable jet. The sheer weight and pressure crushing down on the wood means that no seam of jet has a depth exceeding 50mm.

 

The transformation from wood into jet can only take place in locations which possess a unique geomorphology which allows a chemical shift to occur, the North Yorkshire coastline benefits from perfect conditions in which this ‘jetonization’ process can take place, resulting in the very distinctive black, velvety homogenous appearance of Whitby jet.

Within the cliffs surrounding Whitby, is a timeline of geological rock types which take you on a journey from the oldest Redcar Mudstone to the most recent Cretaceous Chalk. These different periods are visible as layers within the cliffs that make up today’s Dinosaur Coast.

 

The Lower Jurassic rocks (also known as the Lias), are made up of the hardened mud of an ancient seabed that once existed over what is now Cleveland (North Yorkshire) between 204 and 182 million years ago, these are the shales, ironstones and mudstones that sit just underneath the alum shales.

The Lias can be divided into three parts; the Lower Lias (Redcar Mudstone), the Middle Lias (Staithes Sandstone and Cleveland Ironstone) and the Upper Lias (Whitby Mudstone).

The Whitby Mudstone is a very thick formation of up to 60 metres, which is divided into different shale or mudstone members, this includes the Jet Rock Member, and confusingly within the Jet Rock Member is another band called the Jet Rock, a name given by way of eminence to a section around 7 metres thick at the coast and up to 10 metres thick inland, where the very finest jet occurs in seams and lenticular masses that lie parallel with the plane of the strata.

At this level, the jet is very hard, which makes for better definition when carving intricate details, it is also more robust when being shaped on grinding wheels and lends itself very well to being polished, for example, by simply putting a rough piece of jet in your pocket, it will, given a small period of time achieve a very impressive shine.

Good quality jet has been found both above and below the jet rock, however the reliability of material gathered from the jet rock band means it was the sole area for procurement operations by miners in the 19th century.

Fossilisation also needs to take place in sea water. Jet deposits found in fresh water tend to be softer, more brittle and of an inferior quality due to different chemical changes during the fossilisation process which take place in more aerobic conditions than that of sea water.

 

Jet tests from 2.5 to 3.5 on the MOH’s scale of hardness. It has a specific gravity of between 1.15 and 1.18 and is combustible, burning with a light greenish flame and a strong bituminous odour. It cleaves in conchoidal fractures and good quality jet can possess much elasticity as well as being electrically charged in the same way that amber can when rubbed against woollen material. Indeed, such semblance to amber, as well as its similar, organic occurrence led to the Romans calling jet ‘black amber’, a term which continued being used into relatively recent history.

Jet can often be misidentified for other similar organic substances such as Lignite – A poor quality brown coal consisting of obvious woody material, and Cannel Coal – A tough, clean and compact coal material available in large sizes and quantities which can be carved reliably and polished to a dull, black lustre. The matter of determining between jet and these alike materials is often down to the experience and knowledge of Whitby’s jet workers who possess a wealth of experience assessing and working with true Whitby jet.