The Collaborative International Dictionary of English v.0.48:
Jet \Jet\, n. [OF. jet, jayet, F. ja["i]et, jais, L. gagates,
fr. Gr. ?; -- so called from ? or ?, a town and river in
Lycia.] [written also jeat, jayet.] (Min.)
A variety of lignite, of a very compact texture and velvet
black color, susceptible of a good polish, and often wrought
into mourning jewelry, toys, buttons, etc. Formerly called
also black amber.
Jet ant (Zool.), a blackish European ant (Formica
fuliginosa), which builds its nest of a paperlike
material in the trunks of trees.