Sphalerite is one of the most beautiful of all cut gems. It occurs in shades of green, yellow, orange, brown, and fiery red (all colors due to Fe) that are enhanced by faceting. The luster can be adamantine, like diamond, so cut gems with a good polish are very bright, and the dispersion is about four times that of diamond. Consequently, faceted gems are alive with fire and color, which is strong enough to be seen even against the rich body color. Pale-colored or colorless Sphalerite is extremely rare, but gems of the other colors are easily available, as there is no shortage of facetable rough.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Isometric. Crystals widespread, in various shapes; massive, cleavable, granular.|
|Refractive Index||2.50 to 2.396|
|Luster||Resinous to adamantine.|
|Hardness||3.5 to 4|
|Specific Gravity||3.90 - 4.10|
|Dispersion||0.156 (extremely high).|
|Stone Sizes||Gems of hundreds of carats could easily be cut from the large reddish material from Spain. This material is also sometimes cut as cabochons. Green cleiophane from New Jersey has yielded faceted gems as large as 15 carats. Mexican gems could be cut to 50 carats. Smithsonian Institution (Washington, D.C): 73.3, 68.9 (yellow-brown. Utah); 59.5 (yellow-green, New Jersey): 48 (yellow, Mexico); 61.9, 45.9 (yellow, Spain). California Academy of Sciences (San Francisco): 150.3 (dark red-brown oval, Spain). National Museums of Canada (Ottawa, Ontario): 100.1 (dark orange, round, Spain). Private Collection: 24.8 (gray—green, Mt. Ste. Hilaire, Quebec).|
|Luminescence||Bright orange-red to red in LW, SW, from many localities. Material from Otavi, Namibia is triboluminescent.|
|Spectral||Sometimes 3 bands seen in the red at 6900, 6670. and 6510 due to cadmium.|
|Special Care Instructions||See text|
|Formula||ZnS + Fe|
Sphalerite (= BLENDE) dimorph of Wurtzite.
Refractive index (N) and specific gravity plotted against chemical composition in sphaierite, in which Fe substitutes for Zn, in the formula (Zn,Fe)S.
Adapted from W. A. Deer. R. A. Howie. and J. Zussman, 1962, The Rock Forming Minerals, vol. 5 (New York: Wiley), p. 174.
STREAK: Pale brown to colorless.
OPTICS: Isotropic; N=2.37-2.43 (Spanish material 2.40).
OCCURRENCE: Sphalcrite 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.
Wisconsin; Montana; Colorado; Idaho; Arizona. Canada; Tsumeb, Namibia; England; Scotland; Sweden; France; Germany; Czechoslovakia,” Rumania; Australia.
Missouri; Oklahoma; Kansas: so-called Tri-State Region, heavily mineralized by lead and zinc, with many localities and operating mines.
Tiffin, Ohio: red.
Colorado; Utah: may be transparent.
Franklin, New Jersey: almost colorless to pale green, transparent variety known as cleiophane.
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, Zaire; dark green material containing elevated amounts of Co and Fe.
COMMENTS: Sphalerite is one of the most beautiful of all cut gems. It occurs in shades of green, yellow, orange, brown, and fiery red (all colors due to Fe) that are enhanced by faceting. The luster can be adamantine, like diamond, so cut gems with a good polish are very bright, and the dispersion is about four times that of diamond. Consequently, faceted gems are alive with fire and color, which is strong enough to be seen even against the rich body color. Pale-colored or colorless Sphalerite is extremely rare, but gems of the other colors are easily available, as there is no shortage of facetable rough.
Sphalerite, for all its beauty, is too soft and fragile to wear in jewelry. It has dodecahedral cleavage (six directions) and the material is rather brittle and easily scratched. It could be a very important gem if harder and less fragile.
Larger stones (over 20 carats) usually have some inclusions, as well as veils and flaws. so a completely transparent stone is also considered rare. Cutting is all-important, and the appearance of a cut gem depends largely on the quality of the surface polish. Black sphalerite is called marmatite, and the word for sphalerite in European schools is blende.
This dense, beautiful collector stone is found with zinc ores, and has historically been called zinc blende or just blende. With a refractive index somewhat higher and dispersion much higher than diamond, well cut and polished specimens of this gem are breaktaking. Faceting this soft, often color zoned, highly cleavable gem is a challenge few facetors successfully meet. Accordingly, there are relatively few cut specimens available even though rough is not extremely scarce.
The major source of the high iron content, yellow, orange and red specimens is Spain, with greenish and brownish forms coming from Mexico. The sheer beauty of these stones often tempts jewelers and jewelry lovers to set them, and this might be an option if such pieces are worn carefully and rarely. Sphalerite is heat sensitive, fragile and brittle. Cleaning should never be done in an ultrasonic cleaner. All in all, it is best admired as part of a collection.
Fine red or orange sphalerite, especially in sizes above 3 carats is at the top of the value scale. Green, yellow green and brownish stones are less valuable as are smaller pieces of any color. Cut is a strong value factor as few pieces are cut well. Clarity is preferred, and eyeclean or better pieces are preferred, but with a material this scarce in the market some inclusions are tolerated if the color, size and cut are good.
NAME: From the Greek sphaleros, meaning treacherous because it often resembles galena (lead sulfide) but yielded no lead when first smelted. In Europe sphalerite is called blende, from the German blenden, meaning to dazzle. Marmatite, the black variety, is named after the locality at Marmato, Italy.