Gemmological and Spectroscopic Characteristics of Natural and Synthetic Fluorites

Fluorite is not only an important industrial mineral, but also a major species in the jewelry and mineral collection market. In recent years, with the diversification of demand in the gemstone jewelry market, fluorite has been widely used to make faceted gemstones, beads, bracelets, and other forms of jewelry. Fluorite is a common and easy to be synthesized material. Synthetic fluorite sometimes appears in the mineral collections and gem markets. Based on the identification demand of natural and synthetic fluorite, through microscopic observation, femtosecond laser ablation plasma mass spectrometer (fs-LA-ICP-MS), X-ray fluorescence spectrometer (XRF), ultraviolet-visible absorption spectrometer (UV-Vis), infrared spectrometer (FTIR), and photoluminescence spectrometer (PL), inclusion, optical properties, chemical compositions, and spectroscopy characteristics of natural fluorite and synthetic fluorite samples were obtained. Microscopic observation shows that there are no obvious inclusions in synthetic fluorite samples, while natural fluorite samples contain a large amount of gas-liquid inclusions. By fs-LA-ICP-MS, the Nd and Eu contents in synthetic violet fluorite and colourless fluorite samples reach 775.13×10^-6-832.63×10^-6 and 15 173.88×10^-6-16 674.97×10^-6, respectively, significantly higher than those in natural fluorite samples, such amount of Nd and Eu can be detected by XRF and makes it easy for identification. In UV-Vis spectra, synthetic fluorite samples shows obvious absorption and fluorescence peaks caused by doped REE, while natural fluorite samples show nothing obvious in their spectra. FTIR shows a large number of vibration peaks related to CO₃^2-, CO₂, hydroxyl groups, and water in natural fluorite samples, while synthetic fluorite samples do not, which is a useful character to distinguish whether fluorite is natural or not. The PL spectrum indicates that synthetic fluorite has a higher content of various REE than that in natural fluorite, and multiple REE in different valence states produce different fluorescence peaks, resulting in a more complex PL spectrum than that in natural fluorite. Moreover, the Eu²⁺ transition peak intensity of synthetic colorless fluorite samples doped with a large amount of Eu is significantly higher than that of natural fluorite, which can be used to assist in the identification of fluorite's naturalness.