Sinkankas Symposium Showcases Feldspar, from Ordinary to Extraordinary

The following article appears courtesy of Gems & Gemology‘s eBrief

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A group of four labradorite feldspars from GIA’s Edward J. Gübelin Collection: a 30.82 ct cabochon from Finland; a 3.53 ct bicolored green and orange oval cut from Oregon; a 2.60 ct deep orange oval cut from Oregon; and a 4.18 ct light orange oval cut from Oregon. Photos by Robert Weldon.

What do false teeth and moonstones have in common? Or dinner plates and sunstone? How about roofing materials and labradorite? Scouring powder and amazonite?
 
They’re all made from the most abundant mineral group on Earth: feldspar. We walk on it, eat off it, clean with it and, yes, wear it as jewelry.
 
“Feldspars are absolutely everywhere,” said GIA photojournalist Robert Weldon as he introduced the featured gemstone at the eighth annual John Sinkankas Symposium in April. “The Earth’s crust is comprised of between 50 to 60 percent feldspar.”
 
The Sinkankas Symposium, co-hosted by the San Diego Mineral and Gem Society and GIA, was created by Roger Merk to honor the memory of legendary gemologist John Sinkankas, who called feldspar “one of the most fascinating gems known to man.”
 
A silicate mineral made up of silicon, oxygen and aluminum and influenced by other elements, feldspar displays the most gem phenomena of any group, including schiller, labradorescence, peristerescence, cat’s eye and adularescence. That’s what makes them so interesting, Weldon said.
 
“True gem feldspars are few and far between – most of it is just rock,” said GIA researcher John Koivula during his presentation on the microworld of gem feldspars. Most of the time feldspar is merely the matrix holding the gem specimen, which might seem unimportant. “But it can tell you something about the geology and chemistry of the environment that produced the stone,” Koivula said.
 
Also, Dr. Skip Simmons, a researcher at the University of New Orleans, reviewed feldspar’s mineralogy, chemistry and crystallography and talked about his visit to a gem orthoclase deposit in Madagascar. 
 
Lisbet Thorenson, an author and consultant from Beverly Hills who studies ancient gems, described the archaeogemology of amazonite and recent surveys of ancient Libyan and Egyptian mining sites by co-author and geologist Dr. James A. Harrell (University of Toledo).
 
Si Frazier, an author and collector of minerals and gems from Cerritos, California, recounted his trip to Finland in search of spectrolite, which he called “the most beautiful laboradite feldspar type I’ve seen.”
 
Meg Barry, a gem cutter from Fallbrook, California, described feldspar as the “Volkswagen of gemstones – there’s a lot of it, it’s affordable and everyone can have one.” While it may not at first appear to be an exciting stone, she said, it can be cut to be exciting. Berry showed examples of pieces she had recently cut, documenting each step from the beginning rough to the final cut and polished stones.
 
Noted mineral collector Rock Currier, owner of Jewel Tunnel Imports in Baldwin Park, California, related his travels seeking amazonite in Colorado and Ethiopia.
 
Bill Larson, a collector from Pala, California, who travels the world in search of the finest gemstone specimens, shared images of world-class feldspars from Myanmar to Madagascar.
 
Shane McClure, director of identification at GIA’s Carlsbad laboratory, and Dr. George Rossman of the California Institute of Technology in Pasadena addressed the controversy surrounding natural versus treated andesine.
 
“Treatment of feldspar never used to be a serious problem,” McClure said. “But, as has happened with so many gem materials over the past couple of decades, that changed several years ago when gem-quality andesine entered the market.”
 
Researchers began to investigate red and green andesine from China to determine if its color was natural or had been treated, comparing its geologic, chemical and gemological characteristics to known natural labradorite from Oregon and Mexico and untreated yellow andesine from Mongolia.
 
They found that the composition of Tibetan andesine, particularly its anorthite content, overlaps with Mongolian material, but not Mexican or Oregon labradorite. There are no obvious differences in the internal features of Mongolian, Mexican and Oregon material, except for larger copper platelets and potential differences in color zoning seen in some untreated Oregon stones. Material from all three locations has overlapping UV fluorescence. 
 
“Gemologically speaking, the material is very interesting,” McClure said, but GIA knows of no way to reliably separate Tibetan from treated Mongolian stones.  Oregon and Mexican stones can easily be separated from the Chinese using chemistry or sometimes refractive index.
 

Rossman concluded the presentations with evidence that feldspar’s color can be changed through the diffusion of copper. All the samples of red and green feldspar he had analyzed, said to be from Asia and the Congo, were treated by copper diffusion.

 
To learn more about red feldspar, see News from Research.
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