Jet is a product of high-pressure decomposition of wood from millions of years ago, commonly the wood of trees of the family Araucariaceae. Jet is found in two forms, hard and soft. Hard jet is the result of carbon compression and salt water; soft jet is the result of carbon compression and fresh water.
The jet found at Whitby is of early Jurrasic age, approximately 182 million years old. Whitby Jet is the fossilized wood from species similar to the extant Chile pine or Monkey Puzzle tree.