What is Whitby Jet? Whitby Jet information available online is often confusing and more often than not, is actually incorrect. As a professional gemmologist, geologist and postgraduate jet researcher at Durham University Sarah is the only scientist working exclusively in the field of jet research. Her aim is to classify the whole Jet Group of gemstones in order to better understand and ultimately protect these ancient gem materials and those peoples who work them as a way of life. 

The following gemmological data refers to Whitby Jet.

Colour Jet-black
Transparency Opaque using loupe, semi-translucent to a couple of microns using microscope
RI 1.66 (distant vision)
Hardness 3.5-4.0 Mohs equivalent using Vickers Micro-hardness
Inclusions Should be clean but rarely isolated quartz grains. Microscope may reveal a ‘ghost’ tree ring structure
SG 1.3
Lustre Liquid, mirror-like, vitreous
Fracture Dull often semi- iridescent conchoidal in hard Whitby Jet
Streak Light cinnamon brown (gem quality)-chocolate brown (soft jet)
Thermal conductivity Poor
Stability Hard Whitby Jet has excellent resistance to chemical attack so stable in the archaeological record for 5000+ years. Soft Whitby jet stable for 100 years. Can tolerate heat to 200°C


  • Intense Jet black
  • Warm liquid lustre, not the cold black lustre of onyx
  • Poor thermal conductivity – feels body temperature
  • Low SG but feels slightly dense for most plastics
  • Develops a patina, fine network of scratches with lope
  • Dead flat surface free of undulations
  • Should be no woody texture/structures visible with naked eye
  • Whitby Jet never draws black. Beware as modern Russian simulants have slightly lighter streak than Hard Whitby Jet
  • Antique Whitby pieces are all hand carved or faceted so each unique
  • Free of moulding marks
  • Conchoidal fracture often visible on edge of drill hole


What is Whitby Jet?
In simple terms, Whitby Jet is the name given to the fossilized driftwood found in the suite of rocks known as the Mulgrave Shale Member of the Whitby Mudstone Formation. The Whitby Mudstone Formation outcrops between Staithes to the North of Whitby and Ravenscar to the South. The same geology is also exposed in valleys known as inliers within the North Yorkshire Moors National Park, and Whitby Jet was historically mined in this area as well as along the coastal outcrops. These rocks were deposited in the Jurassic Period of geological time making our Whitby Jet 180 million years old.
How did Whitby Jet Form?
The processes that lead to the preservation of wood as jet is not yet completely understood and forms the basis of Sarah’s research, but what we do know is that it is dependent on a very specific set of environmental conditions. During the period of the Jurassic when the Whitby Mudstones were deposited, the area which was to become our modern-day landscape was much closer to the equator and we were located 50-100km offshore in a shallow sea known as the Cleveland Basin. It was a period of global warming. Pressure on the global carbon an oxygen cycle and melting of polar icecaps lead to a situation where wood was preserved in the form of jet and not replaced by crystalline minerals as in the case of the petrified woods we usually see in the geological record. The first time we see these very specific jet creating environmental conditions within the geological record is here at Whitby making our mudstones incredibly important scientifically.
Is it Fossilized Monkey Puzzle Tree?
In regards to ‘The `Monkey Puzzle Myth’ as I like to refer to it, as early as 1933 botanists from Leeds University working with Professor Hemingway on his PhD thesis clearly state that the tree species involved in the production of Whitby Jet are higher-order conifers, definitely not Araucaria araucana – the Monkey Puzzle Tree. This work was confirmed by the Spanish University of Oviedo who identified six different species from three different genus present in Asturian Jet and also Whitby Jet. None were found to be Monkey Puzzle. The scientific information has simply not been accepted, often by others in the jet trade themselves, who believe that folk law sells jewellery!
Is it Coal?
Whitby Jet is a solid hydrocarbon so as such, geologists would usually classify it as a coal. However, for many years coal petrologists have attempted without success to slot jet into the coal classification system. In order to class jet as a coal, we need to say that it is a generic type of coal in its own right. In many ways the previous attempts to classify jet as coal has hindered our understanding of this material from a geological perspective.
Does Whitby Jet only come in black?
The English language has many terms to describe the colour black; as black as pitch, black as coal, even as black as Newgate’s Knocker if you are from London, but there is one term reserved to describe the epitome of blackness, and that is the term “Jet Black”. The true blackness of Whitby Jet is not fully appreciated until the jet is polished, but when it is, an intense blackness is revealed giving the appearance of looking into a bottomless pool of liquid oil. Many of the jets from other sources worldwide do not exhibit the depth of colour seen in Whitby Jet.
What is the difference between Soft and Hard Whitby Jet?
‘Soft’ and ‘Hard’ in regards to Whitby Jet are, as a generalisation Victorian lapidary terms meaning – jewellery grade in the case of Hard, and not so good in the case of Soft. There is in reality negligible difference in hardness from a Moh’s perspective, but there are other gemmological differences exhibited between them Previous literature often suggest that soft jet is deposited in fresh water, hard jet in salt water. Whilst it is generally true that we see soft jet in the deltaic freshwater sequences, we can also find soft jet within the marine shales.
Does Whitby Jet Form in Seams?
Whitby Jet is fossilised driftwood. This means that we could potentially find a whole tree trunk or indeed a tiny twig of jet. Victorian jet miners referred to these outcrops as seems, many of the miners having come from an Ironstone Mining background prior to working as jet miners. We still refer to jet within the cliffs as seam jet, but it doesn’t form in a continuous seam as does coal.
Is Whitby Jet Rare?
Yes, gemmologically speaking jewellery grade Whitby Jet is rare. Only a few significant finds are found in cliff falls each year. These finds only perhaps yielding a few pounds in weight. As mining of the cliffs is not permitted, we must be patient and wait for small pieces of jet to wash in with the tide. This tidal collecting is not an exact science, and it’s not uncommon to find no jet each tide. The rarity of Whitby Jet means that simulants will always threaten our industry.
Are Simulants sold as Whitby Jet?
Yes, the problem of Whitby Jet simulants is as old as humanity. The first simulants of Whitby Jet can be identified in the archaeological record some 4500 years ago in the Bronze Age. In times of high demand, we will always see the demand outstripping supply and the introductions of simulants to the market. Sarah has identified over a dozen Whitby Jet simulants from the Victorian Period alone! Our present-day industry is threatened primarily by Russian, Mongolian and materials from the Georgian Republic which have flooded the market. We are also seeing reconstituted Russian and Chinese material available, often in the style of antique Whitby Jet beads. The danger is that once these materials enter the wholesale pipeline they eventually become Whitby Jet, whether by retailer’s ignorance or by intentional deceit.
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