Therefore, the unique purple-blue shades typical of Yozgat agates are a combination of iron-related colour centres and scattering effect.Ĭolorimetric studies of different light path lengths from a new perspective of UV-Vis spectroscopy. White and blue stripes have grains of about 5 µm and 300 nm in size, respectively, resulting in an accentuated scattering component for the white bands. The perceived changes in band colours are indeed originated by differences in microstructural arrangement and size of the grains visualised through scanning electron microscopy. Accordingly, trace-element composition from laser ablation inductively coupled mass spectrometry confirmed that the two regions have similar Fe content. Peak intensities and shapes are very similar for spectra collected in blue and white bands. In the visible, the broad absorption at about 500 nm, as well as its behaviour at low temperatures, is compatible with the optical activity of iron impurities in quartz matrices, such as that observed in amethysts. Near-infrared diffuse absorption spectra show overtones of hydroxyls vibrations at 1425, 1900, and 2250 nm. X-ray diffraction and Raman spectroscopy revealed the presence of fine grains of quartz and moganite with a preferential accumulation of the latter in the blue bands. In this study, we present a detailed investigation aimed at the identification of the atomic and structural origin of this peculiar colouration of chalcedony. With the increase in the area of peak absorption, the differences in the chroma and colour of the garnet gradually increase in daylight and incandescent light, and it exhibits a more prominent colour-changing effect.Īgates from Yozgat province are appreciated on the gem market for their white and purple-blue banded colours. The absorption bands of Cr3+ and V3+ at 574 nm in the UV-Vis spectra are the main cause of the change in colour. As they exhibit the same capacity to transmit light, the colour of the gem is determined by the external light source. The UV-Vis spectra show two zones of transmittance, in the red region at 650–700 nm and the blue-green region at 460–510 nm. CIE1931 XYZ colour matching functions are used to calculate the colour parameters influencing garnet colour-changing under different light sources. The infrared spectra show that the colour-changing garnets in this paper belong to the solid solution of pyrope-spessartine type. This study examines this species of garnets as well as the causes of the colour change by using infrared and ultraviolet visible (UV-Vis) spectroscopy. A more appropriate heating temperature to obtain citrine by heating amethyst is about 560 ☌.Ī colour-changing garnet exhibits the “alexandrite effect”, whereby its colour changes from green in the presence of daylight to purplish red under incandescent light. When the initial color is darker, the color difference of heated amethyst is larger, and the easier it is to change the color after heat treatment. The color change degree of heated amethyst is related to its initial color. The color at different temperatures can be divided into three stages: The amethyst stage with temperature below 420 ☌, the prasiolite stage with temperature between 420 and 440 ☌ where the color center is the most unstable, the citrine stage with temperature above 440 ☌. The absorption band at 545 nm in the UV-visible spectrum can be related to a charge-transfer transition of Fe3+ and O2-, which has a significant relationship with amethyst lightness and chroma.
The results show that the amethyst color has no significant relationship with cell parameters but the crystallinity index decreases as temperature rises. The effect of heat treatment on amethyst color was studied from a new perspective of chromaticity of gemstones and the cause of amethyst coloration was discussed based on the results of X-ray diffraction, ultraviolet-visible spectroscopy.
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