Yesterday, I wrote about pleochroism and how this optical phenomenon is often misinterpreted as color zoning. As I mentioned, pleochroic gems show different colors when viewed from different crystal directions. So as you move the gem around, the same area of the gem may look a different color when turned in a different direction. This is a direct result of double refraction.
Color zoning is more a result of uneven distribution of color-causing nutrients and conditions in a particular gemstone. Color zoning is found in both doubly and singly refractive stones. With color zoned gems, the stone is actually different colors no matter how you look at it.
Color zoning is an important consideration to gem cutters as it can make or break the beauty of the stone. Some stone cutters will cut a gem with the deepest color at the bottom of the pavilion of the stone, so the color reflects upwards through the entire stone. In cases like this, the color zoning is clearly visible through the pavilion of the stone but through the table you see the reflection of color so the color of the stone looks more consistent than it really is.
Ametrine is a perfect example of a color zoned gemstone. Ametrine is often faceted into rectangular gems with equal distribution of purple and gold. The image below shows an ametrine rough and a faceted ametrine. No matter which way you turn the ametrine, the purple side will be purple and the golden side will be golden. And that’s the difference between pleochroism and color zoning.
Author: Michelle M. Rahm is a GIA-trained Graduate Gemologist and is President of Colorado’s Mile High Chapter of the GIA Alumni Association. She has been selling gemstones and jewelry online since 1997. Visit her websites JewelryImpressions.com and OurCustomWeddingRings.com