Advanced Instrumentation for Identifying Sugar-Acid-Treated Opal
Black opal is commonly more highly valued than white opal because the dark background makes the play-of-colour appear more prominent. Therefore, sugar-acid and smoke treatments have been applied to darken the body colour of light-coloured opal. In addition to its use on white opal, sugar-acid treatment is most commonly done to more porous material such as matrix opal from Andamooka, Australia (Brown 1991; Hsu & Kennedy 2022) and hydrophane opal from Ethiopia (Rondeau et al. 2011). The opal is soaked in a hot sugar solution followed by a hot acid bath, resulting in the deposition of dark carbon material in its porous structure (Brown 1991).
Recently, Guild Gem Laboratory was asked to re-check a 14.30 ct opal mounted in a pendant (Figure 1), since its poor lustre and relatively weak play-of-colour raised concern about an accompanying gemmological report’s claim the stone was untreated opal. The stone’s FTIR spectrum yielded prominent bands at 1115, 782 and 476 cm–1, consistent with opal and ruling out hydrophane, which usually has a main band located at 1095 cm–1 (Hu et al. 2013). Viewed with a gemmological microscope, the pattern of play-of-colour appeared as if it was embedded within a black background (Figure 2a), and reflected light revealed abundant cavities on the surface (Figure 2b). Furthermore, as the stone was tilted, the edges of the play-of-colour pattern did not move as expected.
Figure 1: A 14.30 ct opal mounted in a pendant shows a rather strange appearance, including poor lustre. Photo by Chunyan Wang.
Identification of sugar-acid-treated opal usually involves FTIR spectroscopy and magnification (Brown 1991; Rondeau et al. 2011), but in this case these techniques were not sufficient to confirm whether or not it had been sugar-acid treated, so we applied advanced techniques. The SYNTHdetect instrument was developed for testing diamonds (https://institute.debeers.com/en-gb/instruments/synthdetect), but it has also been used to obtain images of short-lived phosphorescence of other gem materials (e.g. Gao & Sun 2023). In this case, fluorescence imaging of a reference sample of untreated black opal using a SYNTHdetect unit showed a patchy appearance (Figure 3a), while the opal in the pendant exhibited blue luminescence overall except that the black areas were inert (Figure 3b), as expected for carbon material deposited by sugar-acid treatment. This phenomenon could also be seen using a standard UV lamp.
Figure 3: Viewed with a SYNTHdetect instrument, the fluorescence pattern (a) of a 0.67 ct untreated opal appears patchy, whereas (b) the opal in the pendant exhibits overall blue fluorescence with granular-like inert areas corresponding to the dark areas of the stone, indicative of sugar-acid treatment (image width 18 mm). Photos by Wenjing Xu.
A Raman spectrum obtained from the dark spots revealed two prominent peaks at 1330 and 1571 cm–1 (Figure 4), which agree with the characteristic spectral features of amorphous carbon (Beyssac et al. 2002). More than ten additional Raman analyses of such dark areas all indicated amorphous carbon; no graphite was detected. The detection of amorphous carbon with Raman micro-spectroscopy provides another means to identify sugar-acid-treated opal.
SYNTHdetect combined with Raman micro-spectroscopy was found to be an effective and efficient way to identify sugar-acid-treated opal, and can supplement the usual techniques of FTIR spectroscopy and microscopic observation.
Figure 4: The Raman spectrum of dark spots on the opal shows two prominent peaks at 1330 cm–1 (D1 band) and 1571 cm–1 (G band), which are typical of amorphous carbon. The inset photo (by Chunyan Wang) shows one of the areas analysed.
Tiantian Huange, Yujie Gao (peter.gao@guildgemlab.com) and Chunyan Wang Guild Gem Laboratories Shenzhen, China
References
Beyssac, O., Goffé, B., Chopin, C. & Rouzaud, J.N. 2002. Raman spectra of carbonaceous material in metasediments: A new geothermometer. Journal of Metamorphic Geology, 20(9), 859–871, https://doi.org/10.1046/j.1525-1314.2002.00408.x.
Brown, G. 1991. Treated Andamooka matrix opal. Gems & Gemology, 27(2), 100–106, https://doi.org/10.5741/gems.27.2.100.
Gao, Y. & Sun, X. 2023. Gem Notes: Photochromism and phosphorescence of tugtupite. Journal of Gemmology, 38(5), 429–431, https://doi.org/10.15506/JoG.2023.38.5.429.
Hsu, T. & Kennedy, L. 2022. Gem News International: Sugar/heat-treated opal. Gems & Gemology, 58(1), 107–108.
Hu, Y., Fan, L. & Xue, Q. 2013. Study on filling treatment and identification characteristics of opal from Ethiopian [sic]. Journal of Gems & Gemmology, 15(2), 32–37.
Rondeau, B., Fritsch, E., Mazzero, F. & Gauthier, J.-P. 2011. Gem News International: Sugar-acid treatment of opal from Wollo, Ethiopia. Gems & Gemology, 47(4), 333–334.
