Geology & Gemmology

 

Although it is classed as an organic gemstone, Jet it is of a chemical composition very similar to coal, a hydrocarbon, with a maceral content of vitrinite, and not stone-like at all.

Jet is made from the fossilised remains of wood, long believed to be that of the coniferous species of tree, Araucaria Araucana, a modern descendant of this tree is the Monkey-Puzzle. This tree existed in vast forests around the world throughout the Jurassic period, and makes up a proportion of the Whitby Lias rocks.

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.


When an Araucaria tree eventually died it would fall into the surrounding rivers and bogs and slowly make its way towards the sea, breaking up along the way. After becoming waterlogged it would sink to the seabed, where over time layers of sediment, detritus and decaying organisms would bury it, compressing and starving the wood of oxygen under millions of tons of rock, the North Yorkshire coastline benefits from ideal conditions which allow a chemical shift to occur, and over the next 180 to 200 million years the transition would take place from wood into the very distinctive, black, velvety homogenous appearance of Whitby jet.


The sheer weight and pressure crushing down on the tree means that a seam of jet never has a depth in excess of 30mm to 40mm.

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