Micro-World Gems & Gemology, Summer 2018, Vol. 54, No. 2

Wollastonite in Devitrified Glass Imitating Horsetail Inclusions in Demantoid Garnet

Figure 1. A yellowish green glass with numerous wollastonite needles radiating from the center, similar to the horsetail inclusions found in demantoid garnets. Photomicrograph by Ungkhana Atikarnsakul; field of view 14.4 mm.

A 13.12 ct transparent yellowish green gemstone with a beautiful internal scene was examined by the author. The specimen exhibited numerous elongated colorless crystals radiating from the center (figure 1), much like the horsetail inclusions of chrysotile that one would expect to find in Russian demantoid garnets. Standard gemological testing yielded surprising results: a single refractive index of 1.520, specific gravity of 2.50, and medium chalky green fluorescence under short-wave UV light. The specimen was inert to long-wave UV light. These properties indicated that the yellowish green cabochon stone was glass, whereas the masses of needle-like crystals (figure 2) were identified as calcium silicate wollastonite by Raman spectroscopy. Wollastonite inclusions have been reported as an indicator of the devitrification process (Winter 2017 Micro-World, pp. 469–470).

Close-up view of the wollastonite crystals.
Figure 2. Close-up view of the wollastonite crystals. Photomicrograph by Ungkhana Atikarnsakul; field of view 4.8 mm.

Other advanced techniques, including energy-dispersive X-ray fluorescence (EDXRF) and Fourier-transform infrared (FTIR) and Raman spectroscopy, were also carried out to confirm the cabochon’s identity. FTIR and Raman spectroscopy showed a distinct and characteristic pattern of artificial glass. The nonquantitative EDXRF offers no additional value, as the elements detected are also present in demantoid garnets.

Although this is not the first time that wollastonite inclusions have been found in devitrified glasses, they formed in an interesting pattern in this yellowish green gemstone.

Ungkhana Atikarnsakul is an analytics technician at GIA in Bangkok.