A nesosilicate mineral, composition Zn₂SiO₄, crystallizing in the hexagonal system. It is usually massive or granular with a vitreous luster; crystals are rare. The mineral has a basal cleavage and may be variously colored, most commonly green, red, or brown. Hardness is 5½ on Mohs scale; specific gravity is 3.9–4.2. Willemite forms a valuable ore of zinc at Franklin, New Jersey. At this famous zinc deposit willemite fluoresces a yellow-green and is found in crystalline limestone associated with franklinite, zincite, and many other rarer minerals. With the exception of the occurrence in New Jersey, willemite is a rare mineral.