Abstract
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.
Highlights
Fire opal is much sought after for its flame-like red-orange-yellow body-color
Results revealed that the characteristic Ka and Kb peaks of elemental Fe were clearly Results revealed that the characteristic Ka and Kb peaks of elemental Fe were clearly observed (Figure 11a), and the Fe contents were positively correlated with the color satuobserved (Figure 11a), and the Fe contents were positively correlated with the color saturation, which provided important evidence as to the color origin of natural opals
Providedparticles important as to the color origin natural on this,which the dispersed of evidence iron oxides and hydroxides areofthe mainopals
Summary
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. 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. 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. The first derivative of the UV-Vis spectral curves can aid in detecting the use of a dyeing treatment in red and yellow opal. Fire opal is an attractive variety characterized by its intense red-orange-yellow body color and is mainly produced in Mexico. Very little is published on the non-destructive testing of dyed fire opals and their comparison to the natural samples with similar color saturation. The aim of this paper is to provide new reference data and methodological support for the non-destructive testing of natural and dyed fire opals
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