A black, opaque material that takes a high polish. Jet has been used for many centuries for ornamental purposes. It is a compact variety of lignite coal. It has a refractive index of 1.66 (which accounts for its relatively high luster), a hardness of 3–4 on Mohs scale, and a specific gravity of 1.30–1.35. Jet is compact and durable, and can be carved or even turned on a lathe. The principal source is Whitby, England, where it occurs in hard shales. Although popular from Pliny's day until the nineteenth century, the use of jet for jewelry purposes declined markedly for centuries until a resurgence of interest occurred in the 1960s.