Refractometer Guide, Part 1: Refractive Index Testing

Inside the Refractometer

All refractometers function in the same way, though they may look slightly different than the one pictured here.

A rear window allows light to enter. A hinged cover allows you to place a gem inside, on the flat side of a hemicylinder made of special, high RI glass. The viewing lens in front magnifies the scale. The lens has a polarizing filter that can be rotated. Both the filter and the lens are removable.

The gem’s shadow is directed to the scale, and the RI is read where the shadow line falls.

Refractive Index Testing: the General Procedure

To measure the RI of a faceted gem with a refractometer:

• Place a bright light source behind the refractometer and direct it into the light window.
• Place a tiny drop of RI fluid on the hemicylinder.
• Put the gem table-down in the RI liquid (as shown in the illustration) and center it on the hemicylinder.
• Look through the lens. Raise and lower your head until you can see the shadow on the scale. This is your RI.

For more specific instructions on precisely locating the RI, see “Finding the Green…

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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