Garnet has many species because of its common isomorphism. In this study, a suite of 25 natural gem-quality garnets, including pyrope, almandine, spessartine, grossular, and andradite, were examined by standard gemological testing, LA-ICP-MS, FTIR, and Raman analysis. Internal stretching and bending vibrations of the SiO₄-tetrahedra of garnet exhibit correlate with the type of cations in garnet’s dodecahedral position (A site) and octahedral position (B site). FTIR and Raman spectra showed that with the increase of the radius of Mg²⁺, Fe²⁺, Mn²⁺, and Ca²⁺ in A site, or the unit cell volumes of pyrope, almandine, spessartine, and grossular, the spectral peaks of Si–O_str and Si–O_bend modes shift to low wavenumber. Because of the largest cations both in A site (Ca²⁺) and in B site (Fe³⁺), andradite exhibited the lowest wavenumber of Si–O_str and Si–O_bend modes of the five garnet species. Therefore, garnet has correlations between chemical composition and vibration spectroscopy, and Raman or IR spectroscopy can be used to precisely identify garnet species.

Rocks and minerals buried in the earth’s surface usually undergo weathering processes and change color in the burying environment. A kind of yellow Chinese stamp stone named “Lumu stone”, which is buried in a yellowish weathering crust (yellowish soil), was selected to investigate its color changes in the weathering processes. In this study, the appearance features, mineral components, micromorphology, spectroscopy characteristics, and color causation of the “Lumu stone” were studied by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), an electron probe microanalyzer (EPMA), a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), and a UV-Visible (UV-Vis) spectrum. The “Lumu stone” usually exhibits a darker yellow outer layer and a lighter yellow core, suggesting that yellow color permeated into the stone from the surface to the core gradually and the color is secondary forming. The results from XRD and SEM show the studied samples are mainly composed of dickite and illite. The individual particles of the dickite and illite are about 2–5 μm, randomly distributing in the three-dimensional space and constituting voids among the particles. The acid pickling experiments using HCl coupled with KSCN confirmed that the mineral phases that caused the yellow color of the matrix are iron oxide and hydroxide. On the other hand, goethite and hematite were observed gathering in the yellow and brown-red cracks on the “Lumu stone” by SEM study. However, iron oxide and hydroxide in the matrix were difficult to observe and detect among the dickite and illite aggregates by SEM and XRD methods. It indicates that they may be nanoscale in size and very low in content. According to the calculation of the second derivative of Kubelka-Munk (K-M) transformed diffuse reflection spectroscopy (DRS) curves obtained from UV-Vis, the characteristic peaks of goethite and hematite were found in the yellow matrix, and their contributions to the color were confirmed. The concentrations of goethite and hematite were calculated to be 0.32 to 1.87 g/kg and 0.22 to 0.93 g/kg in the studied samples, respectively. In this study, a series of methods were employed to detect very low levels of goethite and hematite in the samples undergoing weathering processes. Additionally, nanoscale goethite and hematite were considered newly formed minerals when buried in the weathering processes and may gradually move into the voids among phyllosilicate particles. Therefore, they turned the “Lumu stone” yellow.

Coesite, a key indicator of ultra-high pressure (UHP) metamorphism, provides critical insights into deep subduction and exhumation processes. However, the deformation mechanisms of intergranular coesite and its retrograde quartz are not well understood. This study investigated the defect structures of shear-deformed coesite and quartz using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) to elucidate their deformation behavior under varying high differential stresses, specifically in the range of 903–1607 MPa. The results indicate that EBSD is ineffective for analyzing defects in materials experiencing substantial plastic deformation. In contrast, TEM analysis visually demonstrates that dislocation density and the prevalence of dislocation tangles increase with rising differential stress in both coesite and quartz. For coesite, hot-pressed samples predominantly exhibited free dislocations and subgrain boundaries, whereas shear-deformed samples displayed dislocation tangles, indicative of enhanced dislocation production. Quartz samples similarly transitioned from free dislocations to dislocation tangles under higher differential stress. Empirical power function relationships between dislocation density (ρ) and differential stress (σ) were established: σ = 3.65 × 10–23 ρ2.39 for coesite and σ = 2.51 × 10–4 ρ0.64 for quartz. These findings elucidate the deformation mechanisms of coesite and quartz under high differential stress, suggesting that the experimental samples may closely reflect the natural deformation processes of intergranular coesite and its retrograde quartz. This study advances the understanding of the role of coesite and quartz in recording tectonic stresses and establishes a TEM-based methodology for estimating differential stress in UHP rocks.

The yellow seal stone from northern Laos is one possible substitute for the Tianhuang Stone, the most famous Chinese seal stone, because of its similar yellow to orange-yellow appearance and the same main mineral composition. The colour causation of the yellow seal stone from northern Laos was studied. The samples’ phase, micro-morphology and chemical components were studied by Raman spectroscopy, and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS), respectively. The yellow seal stone from northern Laos is mainly composed of dickite, occasionally with minor impurity minerals, such as hematite, anatase, barite, diaspore and pyrite. Micro- to nano-sized iron oxides/hydroxides were observed and detected by SEM and EDS in the yellow to orange-yellow part of the samples. Moreover, these iron oxides/hydroxides were suggested to cause the yellow to orange-yellow in the seal stone from northern Laos. The UV-Vis spectrum and its second derivative, the Kubelka-Munk spectra, were used to identify and quantify hematite and goethite. The samples’ colour parameters were obtained with the Commission Internationale de l’Eclairage (CIE) 1931 standard space. According to the observation of the samples and the results obtained from experiments and calculations, the colour of the yellow parts (L* = 33.56~47.99, a* = 0.35~3.65, b* = 4.55~9.89) correlated with goethite (goethite is about 0.175~0.671 g/kg, the content of hematite was too low to be figured out in the yellow parts). In contrast, the colour of the orange-yellow parts (L* = 33.99~46.27, a* = 3.98~12.39, b* = 8.04~22.14) was more closely related with the content of hematite (goethite is about 0.096~0.691 g/kg, hematite is about 0.258~2.383 g/kg). The results of correlation analysis also support that the contents of iron oxides or hydroxides influence the samples’ colour. Therefore, it is suggested that micro- to nano-scaled hematite and goethite caused the colour of yellow and orange-yellow in the studied seal stone. Hematite can strengthen the red hue and change the colour from yellow to orange-yellow.

Characterizes a translucent variety of jadeite jade from the Morales mine in Guatemala.