Emeralds from Riverina, Australia

Figure 1: Emeralds (4.98 – 5.80 ct) from the Riverina deposit in Western Australia, some which possess exceptional gem quality. Photo: L. Phan, SSEF.

by Dr. M.S. Krzemnicki, first published in Facette 26 (May 2020)

In early 2019, an Australian client submitted a small series of gem- quality light green emeralds ranging from 2.01 – 5.80 ct in weight, which reportedly originated from the same rough crystal from the Riverina deposit in Western Australia (Figure 1). Known since the late 1970s (Whitfield, 1975), this deposit has been worked sporadically in the past decades, mostly producing only small amounts of facet grade material.

Found in the dusty outskirts about 6 km north of the Riverina station, this emerald deposit is related to Archean greenschists (chromium-rich metabasalts), cross-cut by Be-rich pegmatite dykes (Stocklmayr et al., 2017), a geological setting well-known from large economic sources such as for example Kafubu in Zambia (Zwaan et al., 2005).

Figure 2a (left): Distinct zoning parallel to prism faces. Figure 2b (right): Hollow channel with brownish Fe-hydroxide and subtle rhombic whitish patches. Photos: M.S. Krzemnicki, SSEF.

The investigated samples – all of rather light green colour – showed very consistent analytical properties and microscopic inclusions. These included very distinct prismatic zonation features (Figure 2a), and hollow tubes parallel to the c-axis with subtle rhombic clusters (probably of tiny fluids) forming whitish patches (Figure 2b). These characteristics have all already been well described by Stocklmayr et al. (2017). The studied Riverina emeralds are rich in iron (Figure 3), with a Cr:Fe ratio of approximately 1:4, resulting in a distinct Fe-related absorption band in the near infrared (± 850 nm, see Figure 4).

  Figure 3 and 4: ED-XRF and UV-Vis-NIR spectra of a Riverina emerald from Western Australia.

Although only rarely found in the trade today, these emeralds from Australia of bright and vivid green colour are a very attractive addition to the treasures this continent down under offers to the gem world.

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