Unique Sphalerite from Spain

Jennifer Stone-Sundberg

Tucson is always full of surprises, and this year was no exception. At the AGTA show, Dudley Blauwet (Dudley Blauwet Gems, Louisville, Colorado) pulled a gem box out of his pocket to show the author a truly unique gem. This yellow sphalerite had a perfect red circle centered in the table with three sharp and distinct black rings encircling it (see above).

The stone was cut from material from Santander in Cantabria, Spain, a source known for producing the best facet-grade sphalerite in the world. The deposit yields orange and red stones that can be quite large, with some weighing hundreds of carats.

Sphalerite, a zinc sulfide (ZnS) mineral, is an important ore of zinc and rarely of gem quality. Colorless in its pure form, it goes from yellow to red to brown to a submetallic black color with the addition of iron. The presence of cobalt can cause a green color. Sphalerite has both an extremely high refractive index (2.37) and dispersion (>0.150), and when properly cut it can have significant brilliance. Despite these desirable gem qualities, sphalerite’s softness (Mohs hardness of only 3.5 to 4.0), brittleness, and perfect cleavage make it a challenge to cut and polish (R.T. Liddicoat, Handbook of Gem Identification, Gemological Institute of America, Santa Monica, California, 1989).

This one-of-a-kind stone was expertly cut in Europe by an artisan who knew how to work soft, brittle stones with cleavage and how to properly center the red “dot” in the culet. The distribution of iron in this sphalerite crystal assumes distinct regions: sharply defined black rings with high iron saturation, a red circular area of medium saturation, and a surrounding area of lower saturation.

This exceptional sphalerite is proof that the combination of Mother Nature and a skilled cutter can reveal the infinite possibilities for spectacular gems.