Vanadium and Chromium-Bearing “Color-Change” Pyrope Garnet

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Pink pyrope garnet seen under incandescent and daylight-equivalent light
An interesting type of pink pyrope garnet, probably mined in Morogoro, Tanzania, is shown under incandescent light A (left) and daylight-equivalent light D65 (right). A noticeable difference in color can be seen. Left to right: a 2.51 ct modified round brilliant faceted by Todd Wacks, a 7.61 ct modified round brilliant faceted by Meg Berry, a 13.09 ct modified round brilliant faceted by Todd Wacks, and 31.90 ct and 6.00 ct rough. The 2.51 ct and 13.09 ct faceted stones were included as part of this study. Photos by Robert Weldon/GIA.

This peer-reviewed feature article—by three of GIA’s young research scientists, Ziyin “Nick” Sun, Aaron Palke, and Nathan Renfro—profiles interesting and attractive Tanzanian color-change garnet that recently reentered the colored gem market at the 2015 Tucson gem shows. The paper is provided here as a PDF download and will be a part of Gems & Gemology’s print and online Winter 2015 issue.

Gem dealers and cutters Todd Wacks, Jason Doubrava, Jeff Hapeman, and Meg Berry provided the samples for the study. According to Wacks, these garnets were mined in Morogoro, Tanzania, around 1988 and were kept a safe deposit box for nearly 30 years. The group was able to obtain the rough three to four months prior to the 2015 Tucson gem shows. After cutting stones from the material, they found the gems showed a striking purple-to-pink color change. Realizing they were unlike any other garnet he’d seen, Wacks resolved to build a market for them. Under fluorescent-equivalent lighting, the larger gems look like fine purple amethyst, while under incandescent they resemble hot pink tourmaline.

This material came to the authors’ attention when Wacks submitted a stone to GIA’s Carlsbad laboratory for analysis. A follow-up with the rest of the group yielded a representative suite of samples for more detailed examination. This study of 32 rough and polished samples up to 31.90 ct provides a comprehensive review of this attractive material’s inclusions along with a quantitative analysis of its color under different lighting conditions. Standard testing showed properties consistent with pyrope garnet, along with the noticeable color difference between incandescent and daylight-equivalent light. A key finding is that larger stones—including two faceted gems of 13.88 and 13.90 ct—appeared to show the most striking “color change.”

Rough stones were fabricated into wafers to orient the inclusions for the best possible photomicrography. The gems were found to contain a wide array of mineral crystals including quartz, apatite, sulfides, and rutile. Raman spectra were collected to conclusively identify these mineral inclusions. Growth tubes, fingerprint-like inclusions, and negative crystals were also observed.

The researchers performed laser ablation–inductively coupled plasma–mass spectroscopy (LA-ICP-MS) on 11 samples to determine their chemical composition. LA-ICP-MS analysis confirmed the garnets were predominantly pyrope (68.92–72.90 mol.%), grossular (4.88–5.61 mol.%), spessartine (9.55–15.08 mol.%), and almandine (5.66–10.14 mol.%), with minor amounts of other garnet species. Compared to more common gem-quality pyrope, this material had higher vanadium and chromium concentration.

Three samples were cut into wafers of various thicknesses for UV-Vis-NIR spectroscopy. This was done to investigate the perception that larger stones, where the light travels a longer through the material, produced the most striking color change. Spectra for the larger gems were extrapolated by multiplying the spectra collected from wafers of known thickness. This allowed the authors to calculate the optimal size for a round brilliant cut from this material to produce the maximum separation between the colors shown under different lighting conditions for the strongest “color-change” effect. The researchers’ colorimetric analysis is informed by the work of physicist and longtime G&G contributor Dr. John Emmett.

“This was an exciting project for us to work on, not only because we got to study such wonderful gem material, but also because we got the chance to hone our skills and adapt our advanced testing methodologies for an in-depth study of color perception,” said Mr. Sun. “For one thing, we had to develop a technique to accurately correct UV-Vis spectra for reflection light loss so we could extrapolate the color of a thin garnet wafer to a faceted stone of a specific size and shape. Understanding the color behavior of this material required us to be able to measure the chemistry accurately. This is where we had to develop our own method to determine the concentrations of chromophores in a complex solid solution like garnet using LA-ICP-MS.”