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.
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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.
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.
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.
Significant gem-quality sources include the following:
Santander, Spain: major gem locality, large cleavages of red-orange color.
Cananea, Sonora, Mexico: fine green transparent material, often pale colored and color zoned, sometimes yellow.
Kipushi, Democratic Republic of the Congo: dark green material containing elevated amounts of cobalt and iron.
United States: Franklin, New Jersey (almost colorless to pale green, transparent variety known as cleiophane); Kansas/Missouri/Oklahoma (so-called Tri-State Region, heavily mineralized by lead and zinc, with many localities and operating mines); Tiffin, Ohio (red color); Colorado/Utah (may be transparent); Arizona; Idaho; Montana; Wisconsin.
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.