“The Star of Asturias,” 163.4 cts, the largest known faceted sphalerite, on display at the Cantonal Museum of Geology, Lausanne, Switzerland, by Sailko. Licensed under CC By 3.0. (Cropped to show detail).
Sphalerite occurs in many colors, including green, yellow, orange, brown, and fiery red. With a dispersion over three times that of diamond and an adamantine luster, faceted specimens make beautiful additions to gem collections. However, they’re too soft for most jewelry uses.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
| Faceted | 1 carat plus |
| to /ct |
| Data | Value |
|---|---|
| Name | Sphalerite |
| Crystallography | Isometric. Crystals widespread, in various shapes; massive, cleavable, granular. |
| Refractive Index | 2.37-2.50 |
| Colors | Colorless (very rarely); black (rich in Fe), brown, orange, yellow, green, orange-red, white-gray. |
| Luster | Resinous to adamantine. |
| Hardness | 3.5-4 |
| Wearability | Poor |
| Fracture | Conchoidal |
| Specific Gravity | 3.90-4.10 |
| Birefringence | None |
| Cleavage | Perfect dodecahedral |
| Dispersion | 0.156 (extremely high). |
| Heat Sensitivity | Yes |
| Luminescence | Bright orange-red to red in LW, SW, from many localities. Material from Otavi, Namibia is triboluminescent. (Produces light under friction). |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long, UV-Short |
| Transparency | Transparent to opaque. |
| Absorption Spectrum | Sometimes 3 bands seen in the red at 6900, 6670, and 6510 due to cadmium. |
| Formula | ZnS + Fe |
| Pleochroism | None. |
| Optics | Isotropic; N = 2.37-2.50. (Spanish material 2.40). |
| Etymology | From the Greek sphaleros, meaning “treacherous,” because sphalerite often resembles galena (lead sulfide) but yields no lead. |
| Occurrence | Sphalerite is the chief ore of zinc, the most abundant zinc mineral, and is common in low-temperature ore deposits, especially in limestones; also in sedimentary rocks; hydrothermal ore veins. |
Sphalerite: Colorado (1.93). Spain (3.30), Mexico (4.65) // Spain (14.48, 5.57). Photo © Joel E. Arem, PhD, FGA. Used with permission.
You’ll find no shortage of facetable sphalerite rough in most colors. Faceting this material, however, poses a challenge. Sphalerites have a low hardness (3.5 to 4) and perfect cleavage.
Iron-rich, opaque, black stones are known as marmatites, named after the locality, Marmato, Italy. In Europe, sphalerite is sometimes called blende, after the German word blenden, “to dazzle.” Though they have no connection to actual ruby gems, stones with red and orange shades are sometimes called “ruby blende.” Low-iron, pale and colorless sphalerites, known as cleiophanes, are extremely rare.
Refractive index (N) and specific gravity plotted against chemical composition in sphalerite, in which Fe substitutes for Zn, in the formula (Zn,Fe)S. Adapted from W. A. Deer, R. A. Howie, and J. Zussman, 1962, Rock Forming Minerals, vol. 5 (New York: Wiley), p. 174.
A number of trace elements may contribute to color in sphalerites. These include germanium, calcium, copper, mercury, and cerium (yellow); tin, silver, and molybdenum (reddish); and cobalt and iron (green).
Sphalerite and wurtzite are polymorphs. They share the same chemical formula (ZnS), but have different crystal habits. Sphalerite has an isometric crystal structure, while wurtzite has a hexagonal structure. These two minerals can coexist in alternating layers as schalenblende, a gem material used rarely for cabochons.
“Schalenblende” by Ra’ike. Licensed under CC By-SA 3.0.
Sphalerites may have a streak color ranging from pale brown to yellow or white. Keep in mind that streak testing can destroy the test sample. Conduct this examination only as a last resort for gem identification on rough, never on a finished gem. This test will also release a sulfurous odor from the test sample.
Sphalerites are pyroelectric. When heated, these gems generate an electrical charge.
Sphalerites may show anomalous double refraction due to crystal strain.
Since sphalerite ranks as the primary ore for zinc mining, scientists have synthesized this mineral for industrial research. In addition, facetable synthetic sphalerites in all their colors have appeared in jewelry and gem collections.
No known gemstone treatments.
Polishing scratches on the bottoms of sphalerite cabochons may cause apparent chatoyancy and asterism. (Although these scratches may occur accidentally, be aware that sphalerites don’t normally display these phenomenal effects).
Significant gem-quality sources include:
“Sphalerite,” Huanggang Mines, near Chifeng, Inner Mongolia, China, 4.1 x 3.0 x 2.3 cm. © Rob Lavinsky, www.iRocks.com. Used with permission.
From the large, reddish material from Spain, faceters could easily cut gems of hundreds of carats, as well as cabochons. Green cleiophane material from New Jersey has yielded faceted gems as large as 15 carats. Mexican material could yield faceted gems to 50 carats.
Clean sphalerites only with a soft brush, mild detergent, and warm water. See our Gemstone Jewelry Cleaning Guide for more recommendations.
“Sphalerite,” Balmat-Edwards Zinc District, St Lawrence Co., New York, USA, 2.75 x 1.75 x 1.5 cm. © Rob Lavinsky, www.iRocks.com. Used with permission.
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