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BACKGROUND: There are differences in the production characteristics, chemical composition, coloration mechanism and formation conditions of green tourmaline from different producing areas. At present, the mineral species and coloration mechanism of Tanzanian green tourmaline have not been solved, which makes it difficult for its scientific identification and quality rating. OBJECTIVES: To study the mineralogical characteristics and coloration mechanism of Tanzanian green tourmaline. METHODS: The mineral component was analyzed by infrared spectroscopy and Raman spectroscopy. Microtexture was characterized by polarized light microscopy. Electron probe microanalyzer, and ultraviolet-visible (UV-Vis) spectroscopy were applied to analyze the chemical composition. RESULTS: The main mineral component of tourmaline in Tanzania was MgO tourmaline, which was characterized by single crystal, small grain, no cleavage, and split development, and can be used as a gemstone. The main chemical component of green tourmaline was SiO2, Al2O3, MgO and B2O3, with average contents of 37.52%, 36.26%, 9.65% and 8.42%, respectively. In addition, it contained a small amount of FeO, Cr2O3 and TiO2. Combined with the absorption spectrum in the ultraviolet region of 440 nm, 600 nm and 680 nm in UV-Vis spectroscopy, it was concluded that the tiny amounts of Cr3+ replacing Al3+ into the crystal lattice was the main cause of the green color. CONCLUSIONS: The mineralogical characteristics and coloration mechanism of green tourmaline in Tanzania was confirmed, which provides a theoretical basis for the scientific identification, further quality evaluation and utilization. [ABSTRACT FROM AUTHOR]

BACKGROUND:Akoya pearl is a biomineralized organic gemstone. The mineral composition and microtextural analysis, and the identification method of color formation of this type of pearl are still lacking. OBJECTIVES:To identify natural gray pearls from treated-color gray pearls. METHODS:The spectroscopic and textural characteristics of two kinds of seawater cultured gray pearls with white or gray nucleus were comparatively investigated by optical microscope, Fourier transform infrared spectroscopy, ultraviolet-visible absorption spectroscopy, and X-ray fluorescence spectrometry. RESULTS:The gray pearls with white or gray nucleus had diameters from 5mm to 9mm, the thickness of 300-500μm. The nacreous layer and its corresponding nucleus of the two kinds of gray pearls were composed mainly of biological aragonite, and their corresponding nacreous layers were rich in Sr and poor in Mn. The pearl with the gray nucleus was rich in Mn. The valence state of Mn was increased due to irradiation, resulting in gray of the pearl core and the whole pearl. In general, the pearls with a white nucleus had an obvious reflection band from 300nm to 400nm in the ultraviolet-visible absorption spectra. On the contrary, those pearls with a light or heavy gray nucleus had a reflection band from 400nm to 500nm. CONCLUSIONS:The two types of gray pearls are all marine cultured pearls. The difference of ultraviolet absorption spectra of the two types of pearls can be used as a basis for judging whether the gray pearls are irradiatedly colored. [ABSTRACT FROM AUTHOR]