What is Jet?

Fossil evidence from the Jurassic era as found on the North Yorkshire coast.Whitby Jet was undoubtedly one of the earliest gemstones used to create artefacts and items of jewellery. The wearing of jewellery was often a display of wealth or used to bestow some 'magical' protection upon the wearer. Because of the exclusivity of many of the raw materials great skill in manufacture and care in design was invested in the items. Whitby Jet scores highly in all the aforementioned areas.

It has a cultural heritage that extends back to early tool making man. This history can be charted from the Bronze age, through the Roman occupation and Viking invasions and onwards to its meteoritic climb to fame in the mid 19th century Victorian England. Its geological history starts in the middle of the Jurassic era, some 150 million years ago at the bottom of the Liassic Sea, which then covered much of our homeland. Fossil evidence from this fascinating era is abundant and easily detected in the cliffs and on the beaches that adjoin Whitby to the North and South. From a literary perspective, Whitby Jet has captured the imagination of many famous authors, the venerable Bede, Drayton, Prior and John Dunne have all used the deep lustrous shine of Jet as a descriptive analogy and the well known phrase "as black as Jet", coined by Shakespeare, is still in common usage.

A Jet Ring Sent
Thou art not as black, as my heart,
Nor half so brittle as her heart, thou art,
What wouldst thou say? Shall both our properties by thee be spoke,
Nothing more endless nothing sooner broke?

Marriage rings are not of this stuff,
Oh, why should aught less precious, or less tough
figure our loves? Except in thy name thou have bid to say,
I am cheap, and naught but fashion, fling me away.

Yet stay with me since thou art come,
Circle this fingers top, which didst her thumb.
Be justly proud, and gladly safe, that thou dost dwell with me.
She that, Oh, broke her faith, would soon break thee.

John Dunne

All this history and cultural heritage is condensed and confined to just seven and a half miles of North Yorkshire coastline! In the light of this it is surprising to find that there has been and still is much conflicting and erroneous information concerning Whitby Jet. It has been called the 'Black amber of the North' a possible allusion to some of the properties that both these fossils share. Even its organic roots have been denied when it was suggested that Jet had nothing to do with wood (Tate and Blake 1876). It has been likened to a mineral deposit similar to turquoise and the most consistent misrepresentation is that it is a by-product of coal formation. The author has even been asked, on more than one occasion, if his Whitby Jet jewellery came in any other colours! Therefore it is my intention to provide a definitive explanation of what Jet is, based upon information drawn from academic geological papers and the collective works of independent authors plus the hands on experience of many years working with this enigmatic gemstone.

The Monkey-Puzzle TreeDuring the mid Jurassic period some 150-180 million years ago what was to be the British Isles was located further south in the area of Northern Spain and Portugal. It was thus much nearer the equator and had a climate to match. The dominant species of tree was the Araucaria very similar to the Araucaria araucana we see today. Its common name is the Monkey-puzzle tree or Chilean pine. On the floor of the sea there had already been deposits of materials that subsequently became the Main Seam Ironstone, on top of this there were deposits of mud being washed down the rivers from adjoining landmass and forming an ever-increasing sedimentary layer. The Araucaria trees that formed a significant part of the vegetable debris were washed into the Liassic Sea and gradually became waterlogged. This served to introduce trace elements not found naturally in the wood and also took them to the bottom of this shallow sea were they became embedded in the thick sedimentary layer.

Thus this new stratum of rock then being formed contained a plethora of these trees, scattered entirely at random. The trees had already received a certain amount of 'pruning' during their journey to the seabed and the huge pressure from the ever-increasing sedimentary layer caused the dispersal of the partly decayed sections of the trees leaving the more resistant sections in situ to gradually become hard Jet. A cliff face showing the Jet shaleThis layer is readily seen as the Jet shale and is dense and composed of fine layers reminiscent of the pages in a book. The average thickness of this layer is between 7-10 metres and other evidence of Jurassic marine life may be found contained within these 'leaves' in the form of several species of ammonite. This shale contains an estimated 0.055 to 0.086 cubic metres of oil per ton and there is also a relatively high percentage of iron pyrite (fool's gold). Some geologists have postulated the future economic importance of this fact and some investigation as to the economic extraction has been done. It has also been suggested that the presence of this oil may have some association with the quality of Whitby jet.

On a personal note, there is a distinctive sulphurous smell redolent of petroleum products that arise from the light ginger-brown dust during the grinding process. On the occasions that pyrites is detected a faint glitter spoils the surface and so is a most unwelcome inclusion. On a wider perspective, the combination of pyrite and oil, present an unstable element and there have been documented occasions of spontaneous combustion in the spoil heaps surrounding some inland mines. In Victorian times there was a great deal of dry grinding on sandstone wheels and the accumulation of dust must certainly have contributed to the frequent and devastating work shop fires.

Pieces of jet showing clear evidence of it's wooden origins.J.E.Hemingway (Leeds University 1933) produced a PhD thesis that gives insight into the 'jetonization' of these trees. The accumulation of mud plus the weight of sea above produced enormous pressures and the individual trees were flattened into narrow 'seams'. The glutinous nature of the sedimentary layer completely sealed these seams and pockets of wood and an anaerobic fossilisation slowly took place. This geological process occurred from the outside in and worked along the medullary rings of the tree. In some specimens of Jet there is a hard stone like centre with a surrounding skin of Jet, this was caused by the centre of the tree becoming silicified before the process of jetonization. However, Jet is usually found in seams ranging from 5mm to 50mm thick and in a variety of lengths. The latter is known as 'plank jet' and the former 'cored jet'. This rare and elusive raw material with which I work is now found as a result of natural coastal erosion and many hours of diligent searching as one can never rely on the sea to give up its harvest when needed! The finest seam jet that I have worked with was found fifteen years ago by my father-in-law at Kettleness point approximately two miles North of Whitby. The seam produced just short of 90 kilos at 40mm thick! It was a truly auspicious find and we have kept a large piece of this seam to remind us of that happy day.

A piece of the 40mm thick jet seam found at Kettleness shown with a British one pound coin (23mm diameter)

In most lapidary texts sparse information is given as to the chemical properties of Whitby Jet. However, 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.

A further note of interest is that with X - ray emission spectroscopy Whitby Jet contains a high proportion of aluminium while Spanish Jet contained more sulphur (Muller 1980) and it has been suggested that this is another contributory fact to the world-renowned quality of Whitby Jet.

Thus to recap:-

Whitby Jet is an anaerobic fossil of the tree Araucaria. The cells of which have been flattened by huge pressures and subjected to complex chemical changes that have taken place over millions of years. Independent seams or pockets are contained in the Jet bearing shale and exposed by the natural erosion of our wild and beautiful coastline.

On top of the Jet shale there is a clearly visible layer of limestone about 100 to 150 mm thick. This is known as the 'Top Jet Dogger' and served as a good guideline for the Victorian miners as very little if any good quality Jet is found above this layer, it also provided a relatively stable 'roof' for their excavations. This light coloured section meanders its way along the cliffs and appears at a variety of levels, sometimes at eye-level sometimes very much higher up the cliffs. Ironically at Whitby it is 5 metres below sea level and does not emerge until Sandsend at the furthest end of Whitby's west side beach, never the less it is still possible to find sea washed pieces of Jet along this part of the coastline.

A further sedimentary layer is composed of Alum bearing shale, which was once extensively quarried and often extended to 37 metres. On top of this is a variable layer of ferruginous rock called the 'Dogger' comprised of sandstone, limestone or sometimes a conglomerate. In the 19th century it was worked as an ironstone band and was referred to as the 'Top seam of Cleveland'. Above this layer there exists an accumulation of rocks that were formed under fresh water conditions. For a time the sea receded which allowed the development of indiscriminate vegetation this produced a thin layer of low-grade coal, which was sometimes mined and is still found on the beaches today. These fresh water conditions gave rise to streams that eventually cut out channels that were then the repositories of 'log jams'. Accumulations of sand and silt buried these and a source of 'soft' Jet was created. This is of inferior quality but was occasionally used in Victorian times for some carvings. The carvings were then set into a large cabochon of the hard and more resistant Jet. Safe to say that the finest work would only contain the very best (3-4 Moh scale) Whitby Jet.

It only remains to mention the final layer of huge sandstone deposits that form the crowning glory of many North Yorkshire hills, Roseberry topping is a fine example close to home. Geology is an esoteric subject and often as 'dry as sticks' but I hope that the passion I have for the material I work with has filtered through and will encourage you to visit the other areas of our web site.