Gem News International Gems & Gemology, Spring 2014, Vol. 50, No. 1

Tenebrescent Irradiated Scapolite

Figure 1. The scapolite pieces on the left side of these photos were irradiated, while the right group of scapolite fragments were not. The photo on the left shows the scapolite before exposure to UV radiation. In the center photo, the scapolite has been exposed to LWUV for 10 minutes. The photo on the right shows the samples after 30 seconds of exposure to SWUV. Scale is in 1 mm increments. Photos by Nathan Renfro.
Sixteen near-colorless scapolite rough crystal fragments, provided by Dudley Blauwet (Dudley Blauwet Gems, Louisville, Colorado) were recently examined in the Carlsbad laboratory. All of the samples were reportedly from the Pitawak mine in the Sar-e-Sang region of Afghanistan’s Badakhshan province. The material was transparent and near colorless, with transparent crystal and fluid inclusions, and some brownish epigenetic staining in cracks.

Half of the crystals had reportedly undergone radiation treatment. The identification of both the natural and irradiated groups as scapolite was confirmed with Raman spectroscopy. Both sets of crystals appeared the same under normal lighting conditions (figure 1, left). When exposed to long-wave ultraviolet (LWUV) light, both the natural and artificially irradiated scapolite showed a strong yellow fluorescence, which was slightly less intense in the irradiated scapolite. Under short-wave UV light (SWUV) a similar intensity trend was observed, with both the natural and artificially irradiated scapolite fluorescing a weak to moderate yellow. However, the most notable difference was in the intensity of the tenebrescent effect exhibited in both sets of stones, which occurs after exposure to both LWUV and SWUV light sources.

Tenebrescence is the phenomenon that occurs when a mineral changes color after exposure to UV light. The color change is temporary, and can be reversed when the material is exposed to incandescent light. Tenebrescence in natural scapolite has been documented (Fall 2005 GNI, pp. 269–271). In the case of these 16 scapolite samples, tenebrescence was observed as a moderately saturated blue color in a previously near-colorless stone. Based on the submitted materials tested, it appeared that artificially irradiating scapolite created a much stronger tenebrescent effect when the material was subsequently exposed to UV light. The material displayed tenebrescence when subjected to LWUV, but the intensity was generally greater after exposure to SWUV (figure 1, center and right). After exposure to SWUV, the visible absorption spectrum of the blue color-causing defect was measured. The resulting visible spectrum confirmed that the blue color was induced in the scapolite (figure 2).

Visible Absorption Spectrum
Figure 2. The visible absorption spectrum of the blue color-causing tenebrescent defect
showed a broad absorption band centered around 610 nm. The transmission window in
the blue region is consistent with the observed blue color.
Irradiation of colorless scapolite appears to have a very limited effect on the material. After performing various gemological tests, the only significant difference we observed was the strength of the tenebrescence, which may serve as a clue that the scapolite was previously irradiated. The appearance of the material was otherwise not improved in any noticeable way.

Tara Allen is a staff gemologist, Nathan Renfro is lead analytical specialist of gem identification, and David Nelson is a metrologist at GIA's laboratory in Carlsbad, California.