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 Viper1
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 Viper1