Gem News International Gems & Gemology, Fall 2013, Vol. 49, No. 3

Unusual Faceted Massive Fuchsite

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Figure 1. This massive Cr-muscovite, reportedly from southern Ethiopia, is better known in the trade as fuchsite. The longest rough piece measures 53 mm, and the faceted stone weighs 2.56 ct. The material was difficult to polish because of its low hardness, 3 or less on the Mohs scale. Photo by Thierry Pradat.
French gem dealer Patrick de Koenigswarter (MinerK) recently showed us some interesting material he purchased in 2011 from an Ethiopian rough dealer. At the time of purchase, the green rough (figure 1) was presented to him as emerald from a new find near the Kenticha pegmatite zone in Ethiopia, well known for tantalite and superb amazonite crystals. However, beryl was ruled out due to the material’s waxy luster and low hardness of 3 or less on the Mohs scale—it was easily scratched with a copper coin.

Routine gemological testing was done on the four samples, which consisted of three rough pieces (17–138 ct) and a 2.56 ct emerald cut. The faceted stone (figure 1, bottom right) always remained lit under crossed polarizers, indicating a polycrystalline structure. The color was unevenly distributed, from light green to intense green, sometimes close to an emerald color. The refractive index, though difficult to measure, was around 1.60. The specific gravity values were 2.80 for two of the pieces and 2.90 for the others, a difference possibly due to the presence of mineral inclusions. All four pieces were inert under both long- and short-wave UV, and red to pink under a Chelsea filter. Additional analyses were required to properly identify this material.

Qualitative and quantitative analyses were performed with a JEOL-5800LV scanning electron microscope (SEM) equipped with a PGT (Princeton Gamma Tech) energy dispersive IMIX-PTS detector. The following elements were found (in atomic %): Si = 16.1%, Al = 17.0%, K = 4.0%, and Na = 1.3%; O was calculated as 61.0%, plus traces of Ba (0.3%), Fe, and Cr. These last two elements were clearly detected on the spectrum but in concentrations too low to be measured (below 0.1%). This composition is consistent with muscovite, a member of the mica family. We observed platy brown polyhedral inclusions, identified by EDS and Raman as rutile due to telltale broad bands around 611 and 443 cm–1.

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Figure 2. The UV-visible spectrum of the Ethiopian fuchsite shows a large absorption band around 600 nm, a continuum of increasing absorption toward the UV, and a weak, sharp peak at 682 nm. These features are consistent with green color produced by trivalent chromium.
We also performed UV-visible absorption spectrometry using a 1 nm sampling and spectral bandwidth in the 350–800 nm range. The spectrum showed a large band centered at about 600 nm and a continuum of increasing absorption toward the UV region (figure 2). These features generated two transmission windows, one in the green around 530 nm and the other in the red to infrared. This explains both the green bodycolor and the red appearance in the Chelsea filter. The continuum is caused by the scattering of light at grain boundaries in this polycrystalline aggregate. We also observed a weak but sharp peak at 682 nm, which we attributed to “chromium lines” of the trivalent chromium (figure 2, inset).

Chemical and spectroscopic results identified Cr3+ as the main cause of the green color. Hence, this material was fuchsite, the chromian variety of muscovite. To the best of our knowledge, this is the first time such a massive, translucent polycrystalline fuchsite has been faceted. The gem’s low hardness makes it unsuitable for jewelry, but it is adequate for collectors.

Thierry Pradat is with Gems-Plus in Francheville, France; Benjamin Rondeau is an assistant professor at the University of Nantes, Laboratoire de Planétologie et Géodynamique, UMR-CNRS 6112; and Emmanuel Fritsch is a professor of physics at Institut des Matériaux Jean Rouxel (IMN) at the University of Nantes in France.