Rare Faceted Thomsonite
The Carlsbad laboratory recently examined a colorless semitransparent 1.04 ct oval modified brilliant (figure 1). Standard gemological testing gave a specific gravity (SG) of 2.47 and a refractive index of 1.520–1.538. These properties suggested the stone was a rare faceted thomsonite, which was confirmed by Raman spectroscopy. There are two types of thomsonite: the more common thomsonite-Ca with the chemical formula NaCa2Al5Si5O20 • 6(H2O) and the less common thomsonite-Sr with the chemical formula of Sr1.4Ca0.6NaAl5Si5O20 • 7.1(H2O). X-ray fluorescence was not run on this stone to determine which, for fear of damaging the potentially fragile stone. The SG reading was slightly higher than the expected value of 2.35 (+0.05/–0.10). Multiple surface scratches and abrasions indicated a relatively soft material, consistent with thomsonite’s hardness of 5.0–5.5 on the Mohs scale. This specimen featured a radiating fibrous inclusion resembling a snowflake (figure 2). Although the identity of the inclusion could not be confirmed with Raman spectroscopy, it was likely a member of the zeolite group.
Thomsonite is a rare member of the zeolite group named for Scottish chemist and mineralogist Thomas Thomson (1773–1852). While the source of this specimen is unknown, thomsonite is typically found in Minnesota along Lake Superior. It usually occurs as semitranslucent to opaque rounded crystal aggregates (figure 1, inset) with a fibrous structure. Thomsonite has also been known to appear as part of a rock aggregate (B.M. Laurs et al., “Benitoite from the New Idria District, San Benito, County, California,” Fall 1997 G&G, figure 13) or as rare needle inclusions in Canadian sapphires (Winter 2004 Gem News International, pp. 344–345). Semitransparent and faceted thomsonite is very unusual. To the best of our knowledge, this is the first faceted thomsonite examined by GIA.
