Hey guys! I've searched far and wide and cannot figure out what Jet is used for (specifically), if anyone knows/can include in the guide, plz do so.
As Mariner said, Jet's only current purpose is to make you some extra money off a kill (ie. you get some gold as a drop, and you sell the Jet to a vendor for some more gold).
That being said, It might not always be vendor trash. There were five stones (Amber, Agate, Jasper, Bloodstone, and Tiger Eye) which were vendor trash until Update 3. Then they became part of an Ancient Statue quest, which put them in relatively high demand. People who had horded their gems in Update 2 made some good money selling to other players.
Update 4 is supposed to have crafting. If jewelry making is a craft, then Jet might have a purpose. Complete speculation, and I am not telling you to horde all your stones... but it is something to consider when making your decision to sell it for a few gold or not.