Evaluation of the Color Changing of Flux‐Synthesized Beryl by Ni Doping Using CIE1976 Color Parameters

Rough beryl contains various transition metal species owing to its geological formation conditions, and their concentrations largely determine the resulting crystal's color—a key physical property that strongly influences gemstone value. However, quantitative relationships between beryl crystal color and concentration of first‐row transition metal content have not been comprehensively reported. In this study, we investigated how Ni concentration affects the color of flux‐synthesized Ni‐bearing beryl crystals, based on CIE 1976 ( L *, a *, and b *) values using micro spectrophotometry. The Ni content was quantified via electron probe microanalysis, while powder and single‐crystal X‐ray diffraction were used to confirm structural characteristics. As Ni concentration increased from 0.00 to 1.61 wt%, the crystal color changed from clear ( m ‐plate: L * = 96.8, a * = –0.368, b * = 0.901; c ‐plate: L * = 95.7, a * = –0.257, b * = 0.773) to moss‐green ( m ‐plate: L * = 79.2, a * = –8.06, b * = 31.5) or deep‐yellow ( c ‐plate: L * = 78.0, a * = –10.8, b * = 81.5). Quantitative relationships between Ni concentration in Ni‐beryl and L *, a *, and b * values, reflecting color variations, were established. This study provides fundamental insights into controlling beryl crystal color through NiO doping or other elements, enabling more flexible tuning of gem color.