The making of jet or mourning jewelry was once a very large industry in
France and England, and even now Whitby jet is well known in commerce.
Jet is a species of bituminous coal (cannel coal) which can be cut with
a knife. The hardness is 1 to 2.5; specific gravity, 1.35; its lustre
is not very high, and color pitch-black.
It is found in England, France, Hesse, Spain, Italy, and Prussia.