Step 3: Practical Gemology

Lesson 4

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…