Polariscope Guide, Part 2: Testing For Optic Figure and Sign


Step 3: Practical Gemology

Lesson 13

Testing for optic figure and sign is an important technique for gemstone identification. You’ll need a polariscope and a condensing sphere. If your polariscope doesn’t have built-in magnification, you’ll also need a hand-held or head-mounted magnifier.

Finding Interference Colors

Begin by setting the polariscope filters to their darkest position. Rotate the gem between the filters and look for interference colors. These are colors that aren’t normal for the gem and will resemble dispersion or the colors of a soap bubble. If you see them, touch your condensing sphere to the area of brightest color. This will reveal the optic figure.

If you have trouble finding the interference colors, look through the longest direction of the girdle while holding your condensing sphere over the girdle. This is a very small area, so you have to move your hand slowly and look carefully. This technique will often reveal an optic sign even when you can’t see interference colors.

Finding The Isogyre

Another technique is to hold the gem upright, rotate it 360°, and, through magnification, observe the stone carefully when it blinks from dark to light. You should see a dark …

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