Lesson 5: Refractometer Guide, Part 2: Cabochon Testing

Step 3: Practical Gemology

Lesson 5

You can use a refractometer to determine the refractive indexes (RI) of most cabochons and carved gems. If your test gems have a flat, polished bottom, your cabochon testing proceeds as with any faceted gem. If not, you can take a spot reading.

Finding a Cabochon Refractive Index

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.

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