Willemite

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Mineral guide

Learn about Willemite

fluorescent zinc silicate with UV glow; daylight colors range from colorless to pistachio, iconic from Franklin–Sterling Hill and Tsumeb.

Regional guide

Willemite from Sterling Hill Mine, New Jersey, USA

Sterling Hill willemite is one of the classic collector materials of American mineralogy: a zinc silicate, Zn2SiO4, that can look modest in daylight and then turn electric green under ultraviolet light. In the best pieces, tan to reddish-brown willemite crystals sit in white calcite with black franklinite and, less commonly, red zincite—an ore assemblage so distinctive that even small fragments announce the Franklin-Ogdensburg district at a glance. The daylight palette is earthy and old-mine: mahogany, brick red, honey brown, buff, white, and black. Under shortwave UV, however, willemite becomes the bright green signal in the rock, while calcite may burn red to orange-red and franklinite stays dark.