Craftsmen who work with jet recognize that material from some areas is much harder than that from others. The “hard jet” tends to be associated with bituminous shales that were deposited in salt water, while the “soft jet” tends to be associated with bituminous shales that were deposited in freshwater environments.

Jet found in the Whitby area of England was deposited in a saltwater swamp about 180 million years ago. It was then compressed during burial and geologically heated to temperatures and pressures that took it through the rank of lignite, and almost to the rank of sub-bituminous coal. This has given the hard jet of Whitby the better working qualities than any other jet that has been found in abundance. As a result, “Whitby Jet” is now famous throughout the world.