Willemite belongs to the phenakite mineral group as the zinc (Zn) analogue to phenakite itself. In fact, willemite has been used as a zinc ore.
Soft, fragile, and difficult to polish, willemites don’t make good jewelry stones. Most faceted specimens show pale green, yellow-orange, or brownish green colors. Rarely, Quebec produces blue material. New Jersey is a well-known source of material for attractive crystal displays and cabochons, including cabs made from troostite, a brown manganese (Mn)-bearing willemite variety, as well as cabs of willemite, black franklinite, and natural red zincite in white calcite. When viewed under ultraviolet (UV) light, these minerals produce quite a combination of vivid luminescent colors and dark inert areas.
Willemites have intense green to yellow-green fluorescence under both shortwave (SW) and longwave (LW) ultraviolet light. Sometimes, they also show intense green phosphorescence. Very rarely, non-fluorescent willemites may occur.
Some willemites may display a weak cat’s eye effect.
Scientists have synthesized willemite for research into various subjects, especially its luminescence. In the early years of fluorescent lighting, manufacturers used synthetic willemite as a phosphor coating for fluorescent tubes. However, there’s no known jewelry use for this synthetic material.
The foremost occurrences of willemite are in Franklin and Sterling Hill, New Jersey. These sites produce stubby green crystals and greenish orange masses to several inches in length, as well as massive troostite material and crystals up to 6 inches long.
Gem cutters have faceted willemites up to about 10 carats in size, mostly from the Franklin, New Jersey occurrence. Cabochons to several inches are also frequently cut from massive Franklin material.
Smithsonian Institution (Washington, DC): 11.7 and 11.1 (yellow-orange, Franklin, New Jersey).
National Museums of Canada (Ottawa, Ontario): 6.75, 0.30 (light blue, Quebec).
Private Collection: 5.39 (pastel orange, New Jersey).
Care
You’re more likely to find faceted or cabbed willemites in mineral collections than jewelry collections. Nevertheless, their striking fluorescence may make them interesting conversation pieces. Since they have a hardness of only 5.5, place them in protective settings to protect them from scratches.
Use a soft brush, mild detergent, and warm water for cleaning. Consult our gemstone jewelry care guide for more recommendations.