Gem News International Gems & Gemology, Summer 2015, Vol. 51, No. 2

Large Cat’s-Eye Emerald from Brazil

Large Brazilian cat’s-eye emerald.
Figure 1. This emerald from Itabira–Nova Era in Minas Gerais weighs approximately 43 ct (approximately 25.30 × 18.10 × 13.75 mm) making it one of the largest Brazilian cat’s-eye emeralds examined by the Gübelin Gem Lab. Photo by Janine Meyer.
Gübelin Gem Laboratories (GGL) in Hong Kong and Lucerne recently had the opportunity to examine a translucent emerald, weighing approximately 43 ct, that exhibited a pronounced and well-centered chatoyancy (figure 1). The stone had a spot RI of 1.57 and a hydrostatic SG of 2.73, and it was inert under long- and short-wave UV radiation. Microscopic observation in reflected and transmitted light presented a series of dense elongated, rectangular, and square multiphase inclusions (figure 2).

Multiphase inclusions in the cat’s-eye emerald
Figure 2. Multiphase inclusions of different shapes and sizes, seen in reflected light (left) and brightfield illumination (right), gave rise to the cat’s-eye effect in the Brazilian emerald. Photomicrographs by Wenxing Xu (left) and Stefanos Karampelas (right). Image width 2 mm (left) and 1.2 mm (right).
The UV-Vis absorption spectra showed the characteristic Cr3+- as well as Fe2+- and Fe3+- related bands. In the FTIR absorption spectra, type I and type II water molecules were observed. An intense band at around 2360 cm–1 indicated the presence of CO2, most likely in the multiphase inclusions. Trace-element analysis of the sample with laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) showed contents consistent with schist- and pegmatite-related emeralds. It also showed lower Li, Cs, and Rb than in emeralds related to highly evolved pegmatites, such as those from Sandawana (Zimbabwe) and Kafubu (Zambia); see J.C. Zwaan et al., “Emeralds from the Fazenda Bonfim region, Rio Grande do Norte, Brazil,” Spring 2012 G&G, pp. 2–17. The stone’s composition was consistent with emeralds from Itabira–Nova Era, Minas Gerais, based on GGL’s reference collection and the published literature on the pegmatites of the area (C. Preinfalk et al., “The pegmatites of the Nova Era–Itabira–Ferros pegmatite district and the emerald mineralisation of Capoeirana and Belmont (Minas Gerais, Brazil): geochemistry and Rb-Sr dating,” Journal of South American Sciences, Vol. 14, No. 8, 2002, pp. 867–887). This exceptional stone is one of the largest Brazilian cat’s-eye emeralds examined by GGL to date. 

Stefanos Karampelas is a scientific researcher, Lore Kiefert chief gemologist, and Wenxing Xu a gemologist, at the Gübelin Gem Lab in Lucerne, Switzerland.