Cabochons of massive brown troostite from New Jersey are attractive, as are cabochons of Willemite with black franklinite and red zincite in white calcite. These latter stones fluoresce vividly in UV light. Faceted willemite is extremely rare and stones larger than 1-2 carats are worthy of museums.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Hexagonal (R). Crystals prismatic, short and stubby or long needles; massive compact; granular.|
|Colors||Colorless, white, gray, various shades of green, yellow, orange, red-brown.|
|Luster||Vitreous to resinous.|
|Density||3.89-4.10 (usually the latter).|
|Cleavage||Poor. Fracture conchoidal. Brittle.|
|Stone Sizes||Faceted gems are known to a maximum size of about 10 carats, mostly from the Franklin, New Jersey, occurrence. Cabochons to several inches are frequently cut from massive Franklin material. Smithsonian Institution (Washington, D.C): 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).|
|Luminescence||Intense green or yellow-green in SW (Franklin, New Jersey), also in LW, sometimes intensely phosphorescent (green).|
|Spectral||Weak bands at 5830, 5400, 4900, 4420 and 4320; strong band at 4210.|
Optics: o=1.691;e =1.719.
Occurrence: In zinc ore bodies or metamorphic deposits where Zn is present.
Greenland; Belgium; Algeria; Zaire; Zambia.
Franklin and Sterling Hill, New Jersey: the foremost willemite occurrence; stubby green crystals and greenish orange masses to several inches in length. Also massive brown material and crystals to 6 inches long, called trooslite.
Inyo County, California ; Utah; Arizona: microcrystals at various localities.
Tsumeb, Namibia: small colorless crystals and bluish masses.
Mt. Ste. Hilaire, Quebec: blue, gemmy crystals.
Comments: Cabochons of massive brown troostite from New Jersey are attractive, as are cabochons of Willemite with black franklinite and red zincite in white calcite. These latter stones fluoresce vividly in UV light. Faceted willemite is extremely rare and stones larger than 1-2 carats are worthy of museums. Most such stones are pale green, yellow-orange, or brownish green, rarely blue (from the Quebec locality). Gems are difficult to polish, and the material is too soft and fragile for use in jewelry.
Name: After King William I of the Netherlands, Troostite after an early American mineralogist, Gerard Troost.