jet 1 
(jĕt)
(jĕt)n.
1. A dense black coal that takes a high polish and is used for jewelry.
2. A deep black.
adj.
1. Made of or resembling a dense, black, highly polished coal.
2. Black as coal; jet-black: jet hair.
[Middle English get, jet, from Anglo-Norman geet, jeet, from Latin gagātēs, from Greek, lignite, jet, after Gagai, a town of Lycia (near present-day Kumluca, Turkey), where pieces of lignite could be found washed out at the mouth of the local river.]