Colouration mechanism of chrysoprase: insights from colourimetry, spectroscopy and mineralogy

Abstract

Chrysoprase, prized for its attractive apple-green colour, has long intrigued gemologists and mineralogists. Although divalent nickel (Ni²⁺) is clearly established as the chromophore, the specific form and structural state of the Ni-bearing phase remain unresolved. This study investigates the colouration mechanism of chrysoprase by assessing the coupled roles of Ni content and crystallinity and identifying the nature of the Ni host phase. An integrated analytical approach was applied, combining instrumental colourimetry, X-ray diffraction (XRD), X-ray fluorescence (XRF), and near-infrared (NIR) spectroscopy study on ten natural chrysoprase samples spanning pale to vivid green colour, with standardised sample preparation (1 mm double-sided polished slices for colour/NIR, powders for XRD), calibrated instruments, and defined measurement parameters (e.g., 4 cm⁻¹ NIR resolution, 0.02° 2θ XRD step size). Colourimetric analysis revealed that chroma correlates positively with Ni content and negatively with crystallinity, with Ni itself inversely correlated with crystallinity. As Ni is predominantly hosted in Ni-bearing phyllosilicates, higher Ni contents reflect greater abundances of these phases, where Ni²⁺ enhances chroma via optical absorption, and the presence of these phases lowers crystallinity. XRD patterns show a broad basal reflection near d ≈ 10 Å, consistent with disordered Ni-phyllosilicates. Moreover, a prominent NIR absorption near 4330 cm⁻¹, attributed to Ni–OH vibrational modes, exhibits partial splitting unique to chrysoprase, reflecting a distorted, less hydrated Ni environment compared with pimelite. Together, these observations demonstrate that the green colour of chrysoprase originates from the poorly crystalline Ni-bearing phyllosilicate intermediate between hydrous, disordered pimelite and well-crystallised, anhydrous willemseite. This work clarifies the mineralogical and spectroscopic basis of chrysoprase's colouration, providing a robust explanation for its distinctive colour.