Gemmological Characteristic of "Tanghe Jade" and Its Quality Evaluation

"Tanghe jade", predominantly harvested from the Tanghe River basin in the northern Taihang Mountain range, is also output in modest quantities along the Juma and Yishui rivers. The primary colours of "Tanghe jade" are white, yellow, red, and black, complemented by secondary colours of gray, cyan, and green. The main mineral component of "Tanghe jade" is tremolite, however, the presence of impurity minerals lead to differences for its quality. In this paper, the methods of combining field investigation and market surveys, the gemmological and mineralogical characteristics of the"Tanghe jade"samples were analysed using conventional gemmological tests (eg.refractive index, relative density, hardness, microscopic observation and infrared spectroscopy), as well as XRD, EPMA, and SEM.The results show that the refractive index of "Tanghe jade" sample is 1.56 to 1.61 and the relative density is 2.85 to 3.07, while the Mohs hardness is between 6 and 7. The "Tanghe jade" is of moderate greasy luster, with wax-oily luster. Polarizing microscope observation results reveal that the predominant mineral component is tremolite, accompanied by impurities such as carbonates, quartz, and diopside. Tremolite exhibits a columnar-fibrous metamorphic texture, with some areas presenting a fibrous metamorphic structure. The fibers are mainly within the confines of 0.01-0.10 mm in length, intricately interlaced to create a tightly woven texture. Some samples exhibit columnar crystals, with grain sizes ranging from 0.05×0.02 mm to 1.00×0.15 mm. These crystals are intricately interlocked, creating a tessellated arrangement, and a minor concentration of impurity minerals is unevenly dispersed throughout the fibrous tremolite matrix.SEM results show that some tremolite particles are distributed in a fibrous, radiating pattern and interwoven together, exhibiting a directional arrangement within small areas. The particles are closely packed, with some displaying a micro-fibrous interlocking structure. Within localized domains, the fibrous tremolite is regularly arranged with closely interwoven particles. Over the entire area, the regularly arranged tremolite particles are interspersed in a non-directional manner. The surface of the tremolite particles is relatively smooth and flat, with a minority of particles exhibiting longitudinal textures. Furthermore, some tremolite particles exhibit the slightly directional distribution, aggregating into bands of varying widths to form collective aggregates. Quartz particles being automorphic form and semiautomorphic form, are interspersed in tremolite, with a particle size of approximately 5 μm. The EMPA and XRD results confirms that tremolite is the predominant mineral of "Tanghe jade", and subordinate minerals encompass calcite, quartz, and diopside. In the context of the tremolite-actinolite series within the calcic amphiboles, the sample's Mg/(Mg+Fe²⁺) ratio surpasses 0.98, converging on the theoretical end-member value of 1.0 for pure tremolite.The infrared spectra of "Tanghe jade" are basically consistent with standard spectral peaks of tremolite, mainly in 600-400 cm^-1, 800-600 cm^-1, 1 200-800 cm^-1, and 3 750-3 600 cm^-1. The distinct and sharp absorption bands near 1 147, 1 063, 1 042, 996 cm^-1 and 922 cm^-1 correspond to the antisymmetric stretching vibrations of the Si-O-Si linkages, as well as the symmetric and antisymmetric stretching vibrations of the O-Si-O bridges. The peaks at 765, 687, 665 cm^-1 and 644 cm^-1 indicate the symmetric stretching vibrations associated with Si-O-Si. Furthermore, the vibrations at 546, 514, 459, 420 cm^-1 are attributed to the bending vibrations of Si-O bonds and the lattice vibrations involving M-O.Additionally, some "Tanghe jade"samples exhibit variable intensity absorptions in the 1 560-1 400 cm^-1, which are caused by the antisymmetric stretching and out-of-plane bending vibrations of the carbonate [CO₃]^2- groups within the mineral's crystalline framework. Based on the above research, the gemmological designation criteria for "Tanghe jade" are established, differentiated by the mass fraction of carbonate minerals. The classification is as follows: (1) when the carbonate mineral mass fraction is below 5% and the tremolite mass fraction exceeds 95%, the material is aptly designated as "tremolite jade"; (2) when the carbonate mineral mass fraction ranging from 5% to 40% and tremolite mass fraction above 60%, the nomenclature is differentiated based on the carbonate composition as either "calcite-tremolite jade" or "dolomite-tremolite jade"; (3) when the carbonate mineral mass fraction reaches or exceeds 50% and the tremolite mass fraction falls below 40%, the material is termed "tremolite marble". By highlighting the distinctive of the pattern floral of "Tanghe jade", along with its vibrant colour palette and diverse textures, and integrating the cultural significance of the "pebbles"with the legacy of the Tang Yao civilization, the targeted promotion and quality-based market positioning of "Tanghe jade" can be strategically directed towards its enhanced development and utilization potential.