The gemstone Jet is black. Jet is a type of lignite, a precursor to coal, and is a gemstone that is formed from fossilized wood. It has been used in jewelry and ornamentation for thousands of years, and was particularly popular during the Victorian era. Jet is often carved into beads, pendants, and other decorative items. It has a smooth, velvety texture and a deep, black color that is often compared to that of black onyx or black glass.