Gem library
Jet
Fossil wood that cuts like a gem
Jet is fossil wood, compressed for a hundred and eighty million years until it became a black material homogeneous enough to be turned, carved and polished to a mirror. It is the lightest gem anyone wears, and it is a coal.
- Hardness (Mohs)
- 2.5 – 4
- Density
- 1.30 – 1.34
- Crystal system
- Amorphous
- Refractive index
- 1.64 – 1.68
- Cleavage
- None
Mineralogical identity
- Family
- Fossil organic matter (lignite)
- Species
- Jet
- Variety
- Hard jet (marine), soft jet (freshwater)
- Chemical formula
- C, H, O (fossilised wood)
- Crystal system
- Amorphous
- Classification
- Organic gem
- Rarity
- Uncommon
- Indicative price
- €0.50 – 20/g
Etymology
From the Greek gagatès, after the town and river of Gagai in Lycia, from which the ancients took the stone. The word gave jet in English and azabache in Spanish.
Physical & optical properties
- Hardness (Mohs)
- 2.5 – 4
- Density
- 1.30 – 1.34
- Refractive index
- 1.64 – 1.68
- Birefringence
- none
- Cleavage
- None
- Fracture
- Conchoidal
- Lustre
- Velvety to vitreous when polished
- Diaphaneity
- Opaque
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- black, very dark brown
Formation, geology & history
From driftwood to jewel
Whitby jet formed in the Early Jurassic, around a hundred and eighty million years ago, from the wood of trees related to the monkey puzzles. Trunks fallen into the sea drifted, became waterlogged, sank and were buried in anoxic mud. Deprived of oxygen they did not rot: pressure and heat drove out the water and the volatile compounds, concentrating the carbon. The result is a very particular lignite, compact, with no apparent grain and no fissures, called «hard» when fossilisation happened in a marine setting and «soft» in fresh water. Only hard jet is really workable; the soft kind crazes as it dries.
Whitby, and Victorian mourning
Jet was known to the Romans and worked in the Middle Ages, but it was the nineteenth century that made it an industry. On the death of Prince Albert in 1861, Queen Victoria wore mourning for forty years, and the court with her: protocol required black, matt jewellery. Whitby, a small Yorkshire port sitting on the deposit, counted up to two hundred workshops and more than fifteen hundred workers in the 1870s. The fashion fell away as fast as it had come, and the industry collapsed at the end of the century. Today's workshops can be counted on one hand, and they work a material that has become rare, cliff extraction being now forbidden.
A sculptor's material
Jet has qualities few gems possess: it is soft, homogeneous, free of cleavage, and it polishes to a deep, warm black very different from the cold black of onyx. It turns on a wood lathe, takes a graver, and drills without chipping. Its lightness, density 1.3, allows voluminous parures that weigh nothing on the neck, which explains the imposing necklaces of the Victorian era. Against that, it scratches easily, it dulls with heat and it can craze if it dries too fast. Well-preserved antique pieces have usually spent their lives in a case.
How to recognise it
- LightnessDensity 1.3: a jet pendant weighs half what an onyx of the same volume does. In the hand the difference is immediate and it is the best clue.
- The brown streakRubbed on unglazed porcelain, jet leaves a chocolate-brown mark, never a black one. Onyx leaves nothing, nor does glass.
- Warmth to the touchIt is warm, like any organic material, where a black mineral stone is frankly cold. Hold it against your cheek.
Imitations, treatments & confusions
- Black glass (French jet)The historic imitation, made from the nineteenth century on. Heavy, cold, and it leaves no streak. Antique glass pieces have their own collector's value, but they are not jet.
- Vulcanite and bakeliteHardened rubbers and resins, much used in mourning jewellery. Vulcanite browns with age and smells of sulphur if rubbed vigorously.
- Ordinary anthracite and ligniteSold as jet. They craze, soil the fingers and do not take a lasting polish.
Deposits & origins
England (Whitby, in Yorkshire: the most famous deposit), Spain (Asturias), France (Aude), United States (Utah, Colorado), Poland, Turkey
Care & precautions
- Never water: it penetrates and the piece crazes as it dries. A soft, dry cloth is enough.
- No heat, no direct sun: jet dries out and cracks.
- Revive the polish with microcrystalline wax, never with an abrasive.
- Store in a cloth, apart: at hardness 3 everything scratches it, including another piece of jet.
Traditions & crystal healing
- Chakras
- Root
- Zodiac signs
- Capricorn · Scorpio
- Element
- Earth
A stone of mourning and protection throughout Europe, worn against the evil eye in Spain in the form of a figa, the closed hand as an amulet, and given to newborns. The link with mourning is historical and social before it is symbolic. Cultural heritage, never medicine.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Jet and onyx, what is the difference?
Everything but the colour. Onyx is a mineral chalcedony, hardness 7, cold and heavy. Jet is fossil wood, hardness 3, warm and very light.
Is Whitby jet still extracted?
Not by mining. Cliff extraction has been forbidden for decades; the workshops work jet picked up on the foreshore after storms, or old stock.
Why has my jet gone matt?
It has been heated or dried. The polish can be revived with wax, but crazing is permanent. Keep it away from radiators.
Tools for this stone
Jet: only the tools this stone actually concerns, each carrying the value read on this page.