Jet is not a mineral. It does not have a crystalline structure, and is a mineraloid. There are many other mineraloids, amber, opal, and pearl, being just three of the better known ones. Whitby jet is a type locality, and is a " hard jet " that was formed during the early Jurassic period by carbon compression of fossil wood in a brackish salt water environment. An article on "Fossil Wood" can be found in issue 1 of the Silurian magazine, which is also on this website.