Natural blue sapphire is a gem species of corundum, which has high economic value due to its beauty and rarity. Color is the most important factor affecting the price of sapphire and mainly caused by trace elements. However, there is little quantitative data currently available to demonstrate the correlation between trace elements and colorimetric parameters of natural blue sapphire. In this study, the UV‐Vis‐NIR spectra and colorimetric parameters were collected by UV‐Vis‐NIR spectrophotometer while the chemical composition was analyzed by laser‐ablation inductively coupled plasma mass spectrometry. The results show that the content ratio of (FeOT + TiO2)/MgO has negative correlation with b* but positive correlation with Cab*, illustrating its obvious effect on the blue color saturation of natural blue sapphire. In addition, the existence of Cr can produce purple hue and the content of Cr shows positive correlation with hue‐angle hab only in the low range (5‐15 ppm), which implies the content of Cr has a limited influence on the hue of natural blue sapphire and the effect may be more complicated as the content of Cr increases above 15 ppm. This investigation can provide quantitative data for scientific color grading and additional applications of natural blue sapphire.
Zultanite is the commercial name of a gem‐quality transparent diaspore that exhibits an obvious color change when exposed to different light sources. It usually appears green under daylight and red under incandescent light. This optical feature makes zultanite valuable, and the study of the spectral characteristics, as well as chemical composition, of zultanite can increase our understanding of the associated color change mechanism. Spectral transmittance curves for zultanite exposed to nonpolarized light emitted by CIE standard illuminants D65 and A show that the color change phenomenon is related to the optical characteristics of light sources. The colorimetric parameters in the CIE 1976 L*a*b* uniform color space system illustrate that the color change exhibited by zultanite is an example of the alexandrite effect, which can be quantitatively described by the significant change of a* and h° parameters. These findings and the results of chemical element analysis by X‐ray fluorescence and X‐ray photoelectron spectroscopy imply that electron transitions between different energy levels involving the 3d orbitals of the trace elements Fe3+ and Cr3+ in zultanite give rise to the relatively high transmittance observed in the green region (around 500 nm) and the orange‐red region (600‐780 nm) of visible light. Thus, the difference in the spectral power distributions of different ambient light sources greatly influence the color of zultanite, making zultanite show green under daylight which is richer in the green light and turns red under incandescent light in which the orange‐red light dominates. Summary The main contributions of this article can be summarized as follows: The colorimetric parameters of zultanite illustrate that its color change phenomenon is an example of the alexandrite effect. The trace elements Fe3+ and Cr3+ are the essential factors that cause the alexandrite effect, which give rise to the relatively high transmittance observed in the green and orange‐red regions of visible light. This article will inspire deeper research on the optical properties and application of hydroxides.
Iron oxides/hydroxides are important magnetic minerals to provide information about changes in the forming environment. However, the magnetic behavior in agate has been rarely investigated. In this study, the magnetic behavior of the Xuanhua-type agate with intense yellow to red colors from the Xuanhua District (China) was investigated by temperature dependence of magnetic susceptibility, hysteresis loop, isothermal remanent magnetization and the analysis of remanent coercivity components from the gradient acquisition plot. Yellow goethite and red hematite can be quantitatively identified by XRD and Raman spectroscopy due to their relatively higher content. Results showed that the red, yellow and orange Xuanhua-type agate had different magnetic behavior, and magnetite existed in the yellow and orange ones. Fluid inclusions in such agate had the homogenization temperature of ~168 °C to 264 °C. All results suggested that the dehydration of goethite to form hematite was the main reason for the high remnant coercivity (above 1000 mT) of hematite in the red agate. The co-existence of magnetite and goethite in the yellow and orange agate reflects the transformation from Fe2+ to Fe3+, indicating the change in the redox property of the environment. Unique patterns mainly formed by hematite and goethite make it a popular gem-material with high research value.
The Chinese large freshwater nucleated pearl has become popular for its unique appearance throughout the international jewelry market in recent years. However, its quality evaluation mostly depends on appearance observations, and the influence of the nacre layer’s internal microstructure on the gemstone’s appearance needs further investigation. In this study, light reflectivity, surface height unevenness parameters and porosity of the nacre layer were measured by chroma meter, laser scanning confocal microscope and X-ray computed tomography (μ-CT), which quantitatively described the characteristics of luster, surface roughness and structure compactness of the nacre layer. It was found that the porosity of the nacre layer had a significant influence on appearance features, with an increase of porosity showing more surface blemishes (higher surface roughness parameters) and weaker luster (lower reflectivity). Related results can provide reference for the scientific and quantitative evaluation of pearl quality.
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