Refractometer Guide, Part 2: Cabochon Testing

Before learning to test cabochons, 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.

Finding a Cabochon Image in a Refractometer

To test a gem with a curved surface:

• Clean the stone thoroughly.
• Next, set up your refractometer without the magnifying lens in place.
• Place a very small amount of RI fluid on the hemicylinder, then carefully place the gem on the fluid, curved surface down.
• If the surface is oblong, place it so the longer direction runs up and down.
• Close the cover if you can. If not, shield the hemicylinder from external light with your hand.
• Move your head up and down until you see the image on the scale.

To get a good spot reading, use the right amount of RI liquid. Your spot should only be two or three scale divisions in length. If it’s more than that, if you have a dark circle around the spot, or if you have a curved cutoff line, you’ve used too much fluid.

It takes practice to learn to…

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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