Whitby Jet is an organic material, naturally formed from fossilised wood.
Millions of years ago in the Whitby area fallen down, dried up prehistoric trees were swept away to the sea, where they sunk deep into the mud. The oxygen-free environment and enormous pressure compacts the wood and over millions of years transforms it into what we know as Whitby Jet.
Whitby Jet is most commonly found in thin seams of shale around 25 – 50mm thick and is formed with a bluish skin around it called spar, which often shows the impressions of fossils. When washed up on the beach jet is usually found without the skin as rounded nugget shaped pieces.
The low specific gravity of jet makes it very light; therefore, even large pieces can be worn easily. It is often carved and takes a high polish. In medieval times, the highly polished jet has been used as a mirror.
The Jet industry became well established in Whitby during the Victorian era. Queen Victoria popularised the wearing of Whitby Jet when she started wearing it as part of her mourning dress following the death of Prince Albert.
The term “jet black” is derived from Whitby Jet’s unique intense black colour.
Resource: https://www.whitby-jet-jewellery.com/what-is-whitby-jet/