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.

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 …

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Content Length 909 words
Reading time 4 min 33 sec

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  • Finding a Cabochon Refractive Index
  • Techniques for Taking a Cabochon RI Reading
    • The 50/50 Reading
    • The Blink Method
    • The Averaging Method
  • Determining Double Refraction in a Cabochon
    • Birefringence Blink
    • Using the Blink to Measure Birefringence
  • The Refractometer Guide Series

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