Step 3: Practical Gemology
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…