Rare Inclusion of Chabazite in a Precious Opal from Ethiopia
An opal received for analysis at the Laboratoire Français de Gemmologie (LFG) in Paris was classified as a light opal, with a bodycolor that matched to neutral gray N8 on the Munsell scale. The play-of-color was divided into sections with well-defined outlines that fit together or were rounded off in the form of digits (B. Rondeau et al., “On the origin of digit patterns in gem opal,” Fall 2013 G&G, pp. 138–146). The drilled bead weighed 12.02 ct and measured 13.4 mm in diameter. It contained small black inclusions of pyrite but also a crystalline inclusion in the form of aggregated pseudocubes (see above). This inclusion was identified as chabazite via Raman analysis. The pseudocubes were in fact near-cubic rhombohedra.
While this mineral has already been documented in veins and opal nodules from Ethiopia, it is rarely seen in cut stones (B. Chauviré, “Genèse de silice supergène sur Terre et implications sur Mars,” PhD thesis, University of Nantes, 2015; N.D. Renfro and S.F. McClure, “Dyed purple hydrophane opal,” Winter 2011 G&G, pp. 260–270). Chabazite, CaAl2Si4O12·6H2O, is a member of the zeolite family. This mineral is formed in gaseous cavities of extrusive volcanic rocks, which is similar to how opals from Ethiopia form. This inclusion is also proof of the Ethiopian geographic origin of this gem, as it has not been described in opal from other localities.
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