Like most colored gemstones, Aquamarine is doubly refractive, meaning light entering the crystal is split into different directions at different speeds due to differences in the atomic structure of the crystal. Rarely is double refraction visible to the naked eye, however, some gems exhibit pleochroism, Aquamarine is one of them.
Pleochroism is an optical phenomenon that is a direct result of double refraction…when a light beam splits and travels in different directions at different speeds, each component of that split beam will absorb different colors. Thus, the gem will appear to have different colors. Pleochroic gems show different colors when viewed from different crystal directions. Aquamarine shows two pleochroic colors…near colorless and strong blue. Pleochroism is an important factor in gem identification.
While I do not have a clear illustration of pleochroism in Aquamarine, the image below shows pleochroism in Kunzite, a variety of Spodumene. Kunzite is known to have moderate to strong pleochroism. Notice how when viewing the crystal through the side, it appears to be purple. But when looking down the length of the crystal, it appears more intense pink to fuchsia. Pleochroism plays a big role in gem cutting as well. Since pink is the more prized color in Kunzite, cutters will orient Kunzite stones with tables perpendicular to the length of the crystal to show more of the pink color.
This image also shows pleochroism in a faceted Blue Sapphire. While they can look similar, be careful not to misinterpret pleochroism to be color zoning. Tomorrow, I will address the differences 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