Professional Gemologist Certification Course
Difficult Gemstone Separations
Scapolite and Citrine
Say you were given a light yellow gem to examine. Your basic refractive index (RI) test yielded two readings on the table 90° apart. The results were 1.545 and 1.552. A polariscope reading confirms what your refractometer results indicated: the stone is uniaxial.
When you review your gemological references, you'll get a long list of possible matches with these characteristics, so you move on to other tests. Other easy-to-find information is the ultraviolet (UV) reaction and pleochroism. First, you find the stone is inert to UV, both long and shortwave. That applies to both scapolite and citrine, so it's no help. You check for pleochroism and find it's weak, pale yellow to yellow. Now the only stone that meets these characteristics is quartz.
The near hits don't include scapolite because the pleochroism for scapolite is moderate to strong. If your eyes are fatigued, it's easy to make this kind of mistake, especially if you're looking at two shades of the same hue.
Another mistake made in difficult gemstone separations is to assume that you've measured the full birefringence of the stone. Scapolite almost always has more...
Donald Clark, CSM IMG
The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”
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