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FIGURE 1: The characteristic colors of Tanzanian tourmaline.


FIGURE 2: Analysis of chrome green tourmaline.


FIGURE 3: Analysis of earth tone tourmaline.
  Unraveling the mystery of the dichroic colors of tourmaline from Morogoro, Tanzania

by Menahem Sevdermish, FGA D.Litt, and Guy Borenstein, FGA EGG
June 17, 2012


During the Tourmaline Day gemological conference that highlighted the 2012 CIBJO Congress in Vicenza, Italy, in May, Menahem Sevdermish, Gemewizard®'s inventor and a recognized expert on the tourmaline family of gemstones, was invited to lecture on the range of colors visible in tourmalines mined in Tanzania. The feedback to his presentation was enthusiastic, and numerous requests were received for transcripts from individuals who were unable to attend the event. The following is a synopsis of the lecture.

The full presentation (in PDF format) can be downloaded here.

The Morogoro area in Tanzania is the source of many gemstones, and most notably spinel, ruby, sapphire and tourmaline. For many years fine quality chrome tourmaline were mined in this and other areas in Tanzania, and during past decade substantial quantities have been produced of fine quality rough tourmaline in earth tone colors, ranging from reddish orange to brown to yellow and green. These highly dichroic gems, show different colors and/or different intensities when observed from different angles, and they were the subject of our color analysis and research.

After purchasing a quantity of earth-tone tourmaline from Morogoro, samples were sent to the Gemological Institute of America in the United States, where they were analyzed as part of a research project conducted together with the University of Oklahoma.

The gems were identified as belonging to the uvite-dravite tourmaline series, and normal tourmaline Refractive Index values of 1.622-1.643 and birefringence of 0.019-0.021 were obtained. Specific gravity was measured as 3.05-3.07. The gems were inert to Long Wave UV light exposure, but did fluoresce, weakly to strongly, chalky greenish yellow under short wave UV. In addition, many of these gems reacted moderately to strongly red through the Chelsea filter.

Gemewizard®'s own research indicates that the specific dichroic color combinations are typical to this area only, and cover a very large portion of the visible spectrum, with the exception of purple, violet and blue.

Figure 1 displays the extent of colors of Tanzanian tourmaline: red to orangey red through yellow and greenish yellow up to bluish green.

We sorted the gems into two main groups: chrome green and earth tones, which we have characterized as "autumnal" tourmaline colors.

Group A in Figure 1 displays the range of colors of the chrome green tourmaline having colors ranging from bluish green to green to yellowish green and Group B displays the earth tone colors, where quite startling color combinations are present through the crown.

To obtain a more in-depth appreciation of the colors involved, we photographed Morogoro gem samples in our light box, under neutral daylight conditions (5500°K). We then examined the images using the Sampler, the color analysis tool in Gemewizard®'s GemePro™ system. Our objective was to define the overall color and analyze the dichroic effect in each of these gems. Unlike our tanzanite analysis in the Gemewizard® Gem Color Report of February 2012, the tourmaline do not require the use of a polarizing filter to identify their pleochroism.

The results obtained from the analysis of the crystal-clear chrome tourmaline in Group A showed three main subgroups: pure green, slightly yellowish green and slightly bluish green.
It was immediately apparent that they had slight to minimal dichroism and usually displayed two shades of the same hue - green.

As is evident in Figure 2, the analysis of the pure green gem showed that 83 percent of the visible color was moderately strong green and 15 percent was light green. The yellowish green gem showed almost equal amounts of light, slightly yellowish green and dark, slightly yellowish green. The bluish green gem showed 67 percent of a combination of two strong bluish green colors and 30 percent of light strong green.

As is visible in Figure 3, strong dichroic color components were apparent in all gems of earth tones, ranging from green to yellow to orange to red. For some of them, the color components were quite different from the overall appearance.

The most distinct dichroic colors were apparent in the brown Red tourmaline. Dark to light yellow, yellowish orange and orangey yellow hues strongly affected the color of these gems as seen through the crown. In other gems, idiosyncratic combinations of green and red colors were clearly visible.

The least distinctive effect was with the yellowish Green tourmaline in the earth tone colors, where 85 percent ratio of dark, slightly yellowish green hues combined with 15 percent of very light yellowish green were obtained. This ratio corresponds with that visible in the chrome green gemstones.

What apparently sets these beautiful earth-toned gems apart from other tourmaline gems is the remarkable combination of dichroic colors seen through the crown. The different color component rays merge together within the gem to display new overall colors.

Another point should be noted. Using the Gemewizard® Sampler, we were able to digitally measure and record, for the first time, the extent of pleochroism in gems. This experiment should pave the way to new research and methodologies of recording color in gemstones.

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