Update on Inclusion Scenes in Mozambican Rubies
In August 2022, a team of GIA gemologists visited the ruby mines near Montepuez in the Cabo Delgado province of northern Mozambique (Fall 2022 GNI, pp. 383–386). This included a visit to the deposits farther west in the region, near the village of Caraia. This mining area is north of the N14 road, while most of the previously known productive ruby deposits are southeast of this highway.
Ruby mining near Caraia has been ongoing for more than a decade but was mostly limited to artisanal mining activity until 2017. In the last few years, the ruby deposits have been developed on a larger scale by Fura Gems. The first official sale of this material took place in late 2021, and regular auctions have been organized since then (Fall 2021 GNI, pp. 276–277).
During GIA’s visit in August 2022, gemologists collected a large suite of rough rubies directly from the wash plant and sort house at the mine site. The samples ranged from 0.1 to 1.2 g and were transparent and fracture-free. The vast majority contained some crystal inclusions, and an extensive inclusion study of this material matched well with the known data from Mozambican ruby, with one exception. The most common inclusions are bands of fine silk, short needles, and particles, including reflective platelets. Mozambican rubies typically show three distinctive types of solid crystals: transparent greenish amphibole crystals with a stubby to elongated shape; pseudohexagonal frosty mica crystals with small expansion fractures; and black opaque sulfide crystals with a metallic luster. All of these features were prevalent in the rubies mined near Caraia.
However, one observation stood out: In 8 of the 85 studied samples, zircon crystal inclusions were found. While zircon crystals are also transparent, their relief is higher than that of the more common amphibole crystals. They also tend to be smaller and are rarely elongated in rubies. The most efficient way to identify them is by viewing the stones between crossed polarizers, which highlights the obvious stress halos around the slightly radioactive zircon crystals (figure 1). In all of GIA’s samples, the identification was also confirmed by confocal Raman spectroscopy.
To our knowledge, zircon crystals have not been previously documented in rubies from Mozambique. They are, however, a hallmark for rubies from Madagascar (A.C. Palke et al., “Geographic origin determination of ruby,” Winter 2019 G&G, pp. 580–613). The presence of zircon crystal inclusions is often a decisive discriminator in the origin determination of rubies. The inclusion scenes of high-quality Mozambican and Malagasy rubies can look very similar, but only the latter were known to have abundant zircon inclusions. This is no longer true now that zircon inclusions have also been identified in rubies from Mozambique.
Trace element analysis can often assist in separating rubies from East African sources. But even there, some overlap occurs between Mozambique and Madagascar, especially for stones with higher iron concentrations. All the Mozambican rubies with zircon inclusions showed a trace element signature that matched completely with other Mozambican rubies when analyzed with both energy-dispersive X-ray fluorescence and laser ablation–inductively coupled plasma–mass spectrometry. Fortunately, the rubies from Caraia tend to have a low iron concentration, reducing the likelihood that their trace chemistry composition will match with Malagasy rubies, which tend to have higher iron concentrations.
Raman spectra of the zircons in rubies from Mozambique (15 inclusions in 6 different stones) and Madagascar (27 inclusions in 13 different stones) showed that the main peak (1015 cm–1) in Mozambican zircon inclusions tends to have lower full width at half maximum (FWHM), but there is no clear separation between the two populations based on the Raman spectra of the zircon inclusions (figure 2).
The discovery of zircon inclusions in Mozambican rubies has a potentially major impact on the origin determination criteria used for the two most common sources of East African ruby. These new discoveries also highlight two important aspects of origin determination research: the critical need to keep collecting samples from known deposits to identify new features in current production, and the power of the microscope as an identification tool in a gemological laboratory.
