Where does the term jet-black come from?
This phrase comes from the stone "jet." Jet is a black stone
used in jewelery, so "jet-black" refers to the color of the stone
Middle English, from Anglo-Norman geet, from Latin gagts, from
Greek, after Gagas, a town of Lycia.]adj.
Made of or resembling a dense, black, highly polished coal.
Black as coal; jet-black: jet hair. Above taken from Dictionary.com