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.
   [1913 Webster]

   Jet ant (Zool.), a blackish European ant (Formica
      fuliginosa), which builds its nest of a paperlike
      material in the trunks of trees.
      [1913 Webster]