Emerald Containing Residues of Green Polishing Compound
Emeralds are commonly treated with organic fillers to reduce the visibility of fissures and thus improve their clarity (McClure et al. 1999), and in some cases green oil is also used to enhance their colour (Ringsrud 1983; Jang‑Green 2016). Recently, while examining a parcel of emeralds submitted for identification, the authors encountered one stone containing organic as well as inorganic material in its fractures.
The green colour of the 2.38 ct gem showed a moderate to high saturation (Figure 22). Its gemmological features identified it as emerald, with RIs of 1.560‑1.568 and a typical FTIR spectrum for beryl. Microscopic observation revealed fluid and mineral inclusions, as well as some distinct areas containing filler in fissures. Transmission FTIR spectroscopy revealed peaks at 2850, 2920 and 2955 cm⁻¹, indicating the presence of an organic filler (i.e. oil; Zwaan et al. 2005). In addition, a green substance in some of the fissures caught our attention (Figure 23a). At first glance, we thought that its greenish appearance was caused by light reflecting off the gem host. However, as we rotated the sample, the substance still appeared green. In addition, it filled several hollow tubes in the emerald (Figure 23b).
Raman spectroscopy of the green substance identified it as chromium oxide (Cr₂O₃), with peaks at 305, 349, 545 and 608 cm⁻¹ (Beattie & Gilson 1970). Chromium oxide is commonly used by gem cutters, usually as a polishing compound or abrasive paste, so we obtained samples of both these materials from the local market in Shenzhen to analyse for comparison (Figure 24). Their Raman spectra agreed well with that of the green substance in the emerald (Figure 25).
The green residues in the fissures of this emerald seem to contribute little to the gem’s colour appearance. Considering the stone’s inherently attractive colour, it is unlikely that the chromium oxide was intentionally introduced in an effort to improve its appearance. The gem cutter may have been using the polishing compound with other gem materials and paid no attention to its green colour when subsequently cutting the emerald. Although such residues are typically removed by cleaning the stone in acid, sometimes they remain in fissures (see, e.g., figure 8 in Ringsrud 1983).
In our view, chromium oxide can be difficult to remove and such a substance in emerald should be disclosed to a client, since it is not intrinsic to the gem and may modify its colour to some extent. A similar situation exists with the presence of green polishing compound within filled fractures and cavities in jadeite. It is possible that not requiring such disclosure may promote the intentional use of green polishing abrasive residues to imitate the appearance of green minerals in jadeite, which play an important role in its pricing.
Dan Ju, Yujie Gao (peter.gao@guildgemlab.com) and Xueying Sun
Guild Gem Laboratories, Shenzhen, China
References
Beattie, I.R. & Gilson, T.R. 1970. The single‑crystal Raman spectra of nearly opaque materials. Iron(III) oxide and chromium(III) oxide. Journal of the Chemical Society A: Inorganic, Physical, Theoretical, 980‑986, https://doi.org/10.1039/j19700000980.
Jang‑Green, H. 2016. Lab Notes: Dyed green beryl. Gems & Gemology, 52(4), 410.
McClure, S.F., Moses, T.M., Tannous, M. & Koivula, J.I. 1999. Classifying emerald clarity enhancement at the GIA Gem Trade Laboratory. Gems & Gemology, 35(4), 176‑185, https://doi.org/10.5741/gems.35.4.176.
Ringsrud, R. 1983. The oil treatment of emeralds in Bogotá, Colombia. Gems & Gemology, 19(3), 149‑156, https://doi.org/10.5741/gems.19.3.149.
Zwaan, J.C., Seifert, A.V., Vrana, S., Laurs, B.M., Anckar, B., Simmons, W.B. et al. 2005. Emeralds from the Kafubu area, Zambia. Gems & Gemology, 41(2), 116‑148, https://doi.org/10.5741/gems.41.2.116.
