Brown opal exhibiting play-of-color has been previously reported (M.L. Johnson et al., “Opal from Shewa Province, Ethiopia,” Summer 1996 G&G, pp. 112–120) and can occasionally be found in the marketplace as “chocolate opal.”
Recently, a 17.15 ct semi-transparent oval cabochon with brown bodycolor and strong play-of-color (figure 1, left) was submitted to the Carlsbad laboratory for identification services. Basic gemological observation and properties confirmed that the stone was opal. The spot refractive index was 1.41. The opal displayed hydrophane behavior by absorbing water, which was easily observed during microscopic examination (Fall 2013 Lab Notes, pp. 175–176).
While this specimen’s properties were consistent with Ethiopian opal with a natural brown bodycolor, it was necessary to confirm that the color was not the result of dye, since this was hydrophane-type material (N. Renfro and S.F. McClure, “Dyed purple hydrophane opal,” Winter 2011 G&G, pp. 260–270). Meticulous examination showed that the brown color conformed to the surface, indicating that it was in fact the result of dyeing after the stone had been fashioned. The uneven bodycolor was difficult to detect because of the stone’s transparency and strong play-of-color. Careful microscopic examination with strong transmitted and diffused white lighting revealed very subtle color concentrations in pits and uneven blotchy brown patches on the surface (figure 2), which confirmed that the brown color was artificially induced. Interestingly, the back of the stone was almost white in the center and brown along the edge (see figure 1, right), showing clear surface-conformal brown color. This suggests that the back of the stone was repolished after dyeing, which would have removed the dark brown rind in the process.
On the brown side, the opal showed an unusual chalky greenish yellow color when examined under long-wave UV light, possibly due to the dye. Advanced gemological analysis with Raman spectroscopy revealed a natural opal spectrum that was free of any peaks pertaining to foreign substances. Energy-dispersive X-ray fluorescence (EDXRF) did not detect manganese, which is the cause of brown color in Ethiopian opal.
Although dyed opal is often easy to identify, this brown dye looked natural, and the color of the stone did not change after hydrophane testing. While the dye was difficult to detect on the top surface of this stone, its back fortunately appeared to have been repolished, offering additional evidence of dye. When examining opal, it is important to consider carefully whether it could be dyed, especially if it is hydrophane material with a color that occurs naturally.