Gemology, Mineralogy, and Coloration Mechanism of Pinkish-Purple Cobaltoan Dolomite from the Democratic Republic of Congo

A pinkish-purple cobaltoan dolomite from the Democratic Republic of Congo (DRC) has appeared on the Chinese gemstone market recently. In this study, Raman analysis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, electron probe micro-analysis (EPMA), scanning electron microscopy (SEM), energy-dispersive X-ray fluorescence (EDXRF), and fiber optic spectrometry were used to explore the gemology, mineralogy, and coloration mechanism of cobaltoan dolomite. Results indicate that cobaltoan dolomite is a mineral aggregate with a granular texture. The degree of fineness and luster differs due to mineral (quartz) impurities in the texture. The cobaltoan dolomites are typically associated with carbonates, such as azurite, malachite, spherocobaltite, and the rare species kolwezite, which were documented in this study. The natural presence of magnesite-spherocobaltite solid solution with distinct chemical zoning occurs as crystals in the country rock. The pinkish-purple dolomites are mainly colored by CoO, whose concentrations ranged from 0.966 to 6.111 wt.%. Based on UV-Vis spectroscopy, cobaltoan dolomite showed broad characteristic absorption bands at 531.5 nm, which varied as the concentrations of CoO increased. The origin of the color is related to the electronic transition of Co2+ and charge transfer between the ions.