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Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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September, 1999

Sphalerite

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 a 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.

Value

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.

Gemological Data:

Makeup: Zinc Sulfide

Luster: vitreous to metallic

Hardness: 3.5 - 4

Crystal structure: Cubic

Fracture: conchoidal to uneven

Cleavage: perfect in 6 directions

Density: 4.09

RI: 2.36 - 2.50 (increases with iron content)

Birefringence: none

Pleiochroism: none

Dispersion: .156

Stones Currently Available:

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