Jet black

Meaning a deep, black colour dates from the mid-15th century and derives from the geological material known as jet, which is part of the lignite group of sedimentary rock of which coal, anthracite etc are members. Jet, when polished, has been used as an ornamental, gemstone-like material from very early times. The word itself derives from 12th century French jaiet, which was eventually anglicised into jet.