Determining Optic Figure and Sign Using a Polariscope

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 carefully observe the stone through magnification when it blinks from dark to light. You should see a dark shadow, called a brush or isogyre, move across the stone as it changes from dark to light. If the entire stone changes all at once, you’re looking straight down the optic axis. Test the gem again, this time…

Donald Clark, CSM IMG

Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book Modern Faceting, the Easy Way.

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