Refractometer Guide, Part 2: Cabochon Testing
Step 3: Practical Gemology
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 …