Step 3: Practical Gemology

Lesson 5

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…