Jet is a precursor to coal. Okay, I have no idea what it means to be a 'precursor' but Wikipedia described it as such. I will tell you this, it looks a lot like coal... very black. It also has an interesting texture that appears as woodgrain, which makes sense considering it is formed by highly compressed, decaying wood. Interestingly, it is lighter in weight than you might expect. Almost like it might float in water.
I mostly use Jet as accent or border material for inlay or intarsia. It works like regular stone and finishes to a high polish.
Clickity here for a more in-depth study of the material.