Refractometer Guide, Part 3: Determining Optic Character and Recognizing Optic Effects

Before learning to determine optic character, be sure to review our article on using a refractometer to determine the RIs of faceted gems. You should be familiar with the instrument, procedures, and testing terminology.

Optic Character

There are five possible optic characters:

• Uniaxial positive
• Uniaxial negative
• Biaxial positive
• Biaxial negative
• Without sign

You can usually ascertain the optic character of a gem from the notes you took while testing it for double refraction. Interpret the results as follows.

Uniaxial

The gem is uniaxial if both refractive indices (RIs) remain the same in each position or if one RI remained the same and the other varied. While doing this, you should have found a position with only one RI.

If you didn’t find a place where there was only one RI, you’ll have to test the stone again on another facet. The gem won’t rest on another facet as easily as it does on its table. You’ll have to hold the facet against the hemicylinder to do this. (Be very careful not to scratch your hemicylinder! Remember to shield it from extraneous light with your free hand, too).

If…

Donald Clark, CSM IMG

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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