Correlating quantified cathodoluminescence spectra in jadeite with micro-scale color measurements in visible-near infrared reflectance spectrometry

Cultures throughout history have valued jadeite jade (hereinafter jade), a natural material assemblage composed predominately of the NaAl endmember pyroxene, jadeite (NaAlSi₂O₆) that is prized for its mechanical properties and enticing coloration [1, 2]. The geological setting for the formation of jadeite-rich rocks and associated complex geochemical phenomena is well documented in the literature [3-5]. Unlike single crystal gems, jade often contains a mixture of colors, but the stone is perhaps best known for exhibiting a range of green hues from pale green to blue green, and to deep green [1, 6]. Efforts to classify jade color include qualitative matching to a set of reference color tiles, and less commonly, use of a more quantitative set of color perception values first established by International Commission on Illumination’s (CIE 1931) color space, which includes values for light-dark luminance level (L*), a green-red vector (a*), and a blue-yellow vector (b*) [7, 8]. Two principal mechanisms have been identified as the cause of green coloration in jadeite: 1) the chemical state of impurity Fe, either 2+ or 3+, and 2) minor to trace Cr in the crystal structure [9, 10]. Previous studies have associated Cr³⁺ with cathodoluminescence (CL) spectral features in jadeite using point spectrometry, with more recent connections made via 2D CL spectrum imaging [11-13]. In this effort, we are seeking to establish whether Cr impurities are correlated with green coloration in jadeite using micro-scale (≤110 μm) visible-near infrared (VIS-NIR) reflectance spectrometry.