Cracking the code of color-changing zultanite
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Figure 1: Zultanite images under different light sources (click to enlarge)


Figure 2: Analysis A (click to enlarge)


Figure 3: Analysis B (click to enlarge)
  Cracking the code of color-changing zultanite

by Guy Borenstein, FGA
February 28, 2012


Responding to the offer we made in the February edition of Color Analysis by Gemewizard® (Analyzing tanzanite color using polarizing filters) that readers send in gem photos for color analysis, Gavin Linsell from Zultgems Inc. (www.Zultgems.com) contacted us and proposed one of his company's gems - a magnificent 12.32-carat octagon shaped, mixed cut zultanite.

Zultanite is a rare color-change variety of diaspore found exclusively in Turkey. It is named for the sultan rulers who used to govern the region in the Anatolian mountains where it is sourced. Zultanite presents noticeable pleochroism and color-change phenomena, which produce selective absorption of color, depending upon the orientation of the stone and the frequencies of the light waves. When placing zultanite under different light sources, it displays a variety of body colors that are specific to each one of them, ranging from kiwi-green to canary yellow to raspberry red. These colors can be seen also from different angles.

When holding a zultanite gemstone in your hand, you will find it hard to describe its body color. The variety of colors produced by the gem and the distinct differences between them make it impossible for a single color description. But the GemePro™ analysis provided us with a practical solution. By locating the dominant color components of the gem, we could give a multi-color identification, and, by that, a more accurate definition. Laboratories and appraisers issuing certification documents, as well as jewelers and dealers describing their gems, can use multi-color definitions like this, when needed, to achieve a clearer understanding and definition of the gems' color compositions.

For the purpose of this analysis, the selected zultanite gem was photographed from three directions under four different light sources (Figure 1): fluorescent "daylight" (6500°K), incandescent light (halogen 3200°K), typical indoor light (80% fluorescent, 20% incandescent) and mixed light (50%-50%, which is supposed to represent outdoor white light). For each light source, we used the resultant images to analyze the color mixture displayed, by breaking it into its main color components and retrieving the average. Next, we compared the components' ratio between sources to check for similarity and consistence.

The images taken under fluorescent-only and incandescent-only lighting (Figure 2) showed a fairly accurate average color. Each average color assessment was achieved by combining three main color components. The hue difference between the components, according to their position in the GemeSquare™'s 31 main hues, ranged from one place in the incandescent lighting to four place in the fluorescent.

The mixed and indoor lightings analysis (Figure 3) did not retrieve the true representative average colors, due to the mixture of the colors produced by the gem. The outdoor lighting analysis showed four dominant color components covering the areas of yellow and orange. We believe that if the analysis parameters were set to look for smaller color groups, using higher resolutions, it would also find green and red-colored groups in this image.

Surprisingly, the indoor lighting image analysis identified three color components, as in the separate fluorescent and incandescent lightings. Moreover, the ratio between the components was similar to the ratio in the separate lighting analysis.

FluorescentIncandescentIndoorApproximate color component range
Dominant color component #140.40%42.07%45.65%~40-45%
Dominant color component #232.32%38.62%34.78%~32-38%
Dominant color component #327.27%19.31%19.57%~19-27%


If you have a magnificent gemstone or colored diamond and would like Gemewizard® to analyze it in one of its next Gem Color Reports, please contact us at info@gemewizard.com.