Dramatic Color Zoning in Alexandrite
![Change of color observed in an alexandrite.](https://www.gia.edu/images/FA20-LN-Fig1-261214-636px.jpg)
An 8.55 ct alexandrite was recently submitted to GIA’s Bangkok laboratory. Standard gemological testing yielded a refractive index of 1.745–1.754 and a specific gravity of 3.73. The stone displayed a change of color from green in daylight to reddish purple in incandescent light (figure 1). All of these properties were consistent with alexandrite.
![Color zoning in alexandrite when immersed in methylene iodide.](https://www.gia.edu/images/FA20-LN-Fig2AB-261213-636px.jpg)
Microscopic examination revealed natural fingerprints, needles, zoned particles, and color zoning, along with internal growth features. Interestingly, the stone showed a thin color zone very close to some of the pavilion facets and culet when immersed in methylene iodide (figure 2).
Testing with a handheld spectroscope and ultraviolet-visible (UV-Vis) spectroscopy revealed chromium (Cr3+) features. The absorption feature assigned to Cr3+ was located at 680 nm, with a broad band around 589 nm (figure 3). In contrast, when examined from the side (girdle to girdle), no Cr3+ feature was detected. This confirmed that color zoning at the culet strongly affected the color of this stone and its color-change effect.
The cutter designed the gem to contain the thin color zone along the pavilion facets and culet to display a color change when viewed face-up. Without the Cr3+-containing color zone, this stone would be chrysoberyl.
This example illustrates that orienting a color zone with the right trace elements combined with proper cutting can affect the variety designation of a gem.