Willemite is a zinc (Zn. It is highly fluorescent (green) under shortwave ultraviolet light. It occurs in a variety of colors in daylight, in fibrous masses, solid brown masses (troostite), and apple-green gemmy masses.
It was discovered in 1829 in the Belgian Vieille-Montagne mine. Armand Lévy was shown samples by a student at the university where he was teaching. Lévy named it after William I of the Netherlands (it is occasionally spelled villemite). The troostite variety is named after Dutch-American mineralogist Gerard Troost