Willemite Value, Price, and Jewelry Information
Willemite is prized for its intense green fluorescence. Too fragile for jewelry use, faceted specimens are extremely rare.
Willemite Value
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
Willemite Information
| 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. |
| Fracture | Conchoidal |
| Hardness | 5.5 |
| 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). |
| 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 (+). |
| 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. |
Comments
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.
Identifying Characteristics
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.
Synthetics
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.
Enhancements
No known gem treatments or enhancements.
Sources
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:
- United States: Arizona; Inyo County, California ; New Mexico; Utah. (Micro-crystals at various localities).
- Algeria; Belgium, Democratic Republic of the Congo; Greenland; Mexico; Zambia.
Stone Sizes
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.
