Step 3: Practical Gemology

Lesson 27

While some gems are fairly easy to distinguish, others present greater challenges to gemologists. Two of the most difficult gemstone separations to make are scapolite from citrine and apatite from tourmaline. The good news is there are tricks to telling them apart. Both separations begin with a thorough examination.

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 degrees apart. The results were 1.545 and 1.552. A polariscope reading tells you 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 short wave. 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…