Color formation and illumination-dependent color variation of chrysoprase revealed by Kubelka-Munk optical analysis

Chrysoprase is a translucent green variety of chalcedony whose color plays a key role in gemological evaluation and visual presentation but is highly sensitive to illumination owing to complex internal light transport. However, a quantitative optical understanding of how absorption and scattering jointly influence color characteristics and illumination-dependent color variation is required. In this study, ten double-polished chrysoprase thin sections were analyzed using transmittance and reflectance measurements. CIE color coordinates under illuminants A, D65, and F11 were calculated from transmittance spectra, while effective absorption and scattering coefficients were derived from black-white background reflectance using the Kubelka-Munk method. Elemental compositions were determined by energy-dispersive X-ray fluorescence, and illumination-dependent color variation was quantified using the CMCCON02 color inconstancy index based on CAT02 chromatic adaptation and CIEDE2000 color difference. The results show that higher NiO content is strongly associated with higher effective K values in the violet-blue and red regions and with greater chroma, whereas hue is closely associated with the wavelength position of the minimum in the effective K spectrum within the green region. Nickel content also correlates positively with the effective scattering coefficient, while translucency-related background sensitivity is associated with the combined effects of absorption and scattering. Color inconstancy showed strong associations with absorption-related parameters. In particular, the variance of the wavelength-dependent effective absorption coefficient K(λ) showed strong positive correlations with the CMCCON02 indices, although these relationships may partly reflect the common influence of NiO content and overall absorption magnitude. This study provides a quantitative optical framework linking absorption-scattering properties to color and illumination sensitivity in chrysoprase.