Comparison of Natural and Dyed Fire Opal

Fire opal is much sought after for its flame-like red-orange-yellow body-color. With consumers’ enthusiasm for fire opals, dyed fire opal has gradually entered the jewelry market, which has made an impact on consumer perception and trade. In this work, we present a combined spectroscopic (FTIR, Raman, UV-Vis) investigation and chemical analysis of four natural fire opals from Mexico and Ethiopia and four dyed samples. Ultra-depth microphotographs revealed uniform color distribution in the natural fire opal, while patches of color were observed in the dyed fire opal. The main infrared peak around 1099 cm−1 indicated the hydrophane origin of all the dyed materials. The color of natural fire opals is related to the presence of Fe oxides and hydroxides (hematite, goethite) as confirmed by the Raman spectra, the first derivative of the UV-Vis spectral curves, and the relatively high Fe content by chemical analysis. By contrast, dyeing opals created several Raman peaks produced by external materials, and an extremely low Fe content was detected. The first derivative of the UV-Vis spectral curves can aid in detecting the use of a dyeing treatment in red and yellow opal.