Brownish orange-yellow willemite, custom shield cut, 5.59 cts, 11.7 x 9.6 mm, Tsumeb, Namibia. © The Gem Trader. Used with permission.
Willemite is prized for its intense green fluorescence. Too fragile for jewelry use, faceted specimens are extremely rare.
| Data | Value |
|---|---|
| Name | Willemite |
| Crystallography | Trigonal/Hexagonal (R). Crystals prismatic, short and stubby or long needles; massive compact; granular. |
| Refractive Index | 1.691-1.719 |
| Colors | Colorless, white, gray, various shades of blue, green, yellow, orange, red-brown. |
| Luster | Vitreous to resinous. |
| Hardness | 5.5 |
| Fracture | Conchoidal |
| Specific Gravity | 3.89-4.10 (usually the latter). |
| Birefringence | 0.028 |
| Cleavage | Poor |
| Luminescence | Intense green or yellow-green in SW (Franklin, New Jersey), also in LW and X-ray, sometimes intensely phosphorescent (green). |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, Phosphorescent, UV-Long, UV-Short, X-ray Colors |
| Transparency | Transparent to opaque. |
| Absorption Spectrum | Weak bands at 5830, 5400, 4900, 4420 and 4320; strong band at 4210. |
| Phenomena | Chatoyancy (rare). |
| Formula | Zn2SiO4 |
| Pleochroism | Dichroism variable. |
| Optics | o = 1.691; e = 1.719. Uniaxial (+). |
| Optic Sign | Uniaxial + |
| Etymology | Willemite after King William I of the Netherlands; troostite after an early American mineralogist, Gerard Troost. |
| Occurrence | In zinc ore bodies or metamorphic deposits where Zn is present. |
Willemite, New Jersey, 1.02 cts. Photo © Joel E. Arem, PhD, FGA. Used with permission.
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.
A cherry-red zincite crystal on a matrix of calcite, willemite, and franklinite. Under UV light, the zincite and black franklinite are inert, while the calcite fluoresces reddish orange and the willemite bright green. 3.1 x 1.9 x 1.5 cm, Sterling Hill, Ogdensburg, Franklin Mining District, Sussex Co., New Jersey, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
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.
Greenish yellow cat’s eye willemite, round cabochon, 8.56 cts, 11.1 mm, Franklin, New Jersey. © The Gem Trader. Used with permission.
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.
No known gem treatments or enhancements.
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.
A large troostite crystal on a calcite matrix, under white and UV light. 14.0 x 13.0 x 11.0 cm, Franklin, Sussex County, New Jersey, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
Mt. Ste. Hilaire, Quebec produces blue, gemmy crystals.
Tsumeb, Namibia also produces facetable crystals as well as masses.
Other notable sources include the following:
Willemites with unusual tentacle-like shapes. 9.0 x 8.4 x 5.0 cm, Potosi Mine, West Camp, Santa Eulalia District, Chihuahua, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.
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.
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.
Willemite: Franklin, New Jersey (0.5, 6.94, 0.6). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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