gems, jade, amphibole jade, nephrite, China, Qinghai jade

At present, more and more black nephrite jade has appeared in both domestic and international markets, with much of it originating from newly discovered deposits in the Mo ore region of Qinghai. Using modern analytical techniques, the gemological and mineralogical characteristics of black nephrite from Qinghai have been systematically studied, and its genetic type has been inferred based on geological context. Polarizing microscope observations show that the main mineral in Qinghai black nephrite is tremolite, with numerous internal filling cracks and inclusions of graphite, magnetite, ilmenite, and other mineral impurities. Scanning electron microscopy reveals that its primary texture is columnar, with microstructural features including fibrous interwoven textures and scaly metacrystic structures. Infrared spectroscopy of representative samples shows absorption peaks corresponding to tremolite, with the peak positions unaffected by color variation. X-ray diffraction analysis confirms that Qinghai black nephrite closely matches the unit cell parameters of standard tremolite. Differential thermal analysis shows an endothermic effect during heating to 1000 °C, attributed to the loss of interlayer and structural water, with weight loss ranging from 1.315% to 5.058%. Electron probe analysis yields Mg²⁺/(Mg²⁺+Fe²⁺) ratios above 0.9, confirming classification within the tremolite series. The formation of color in the nephrite does not appear to be related to typical chromophoric elements such as Ti, Mn, Cr, or Fe, and no clear correlation was found between color and the substitution of Mg–Si, (Ca+Na+K)–Al, or (Ca+Na+K)–Fe in the crystal structure. Comparative analysis with black nephrite from Hotan and Xiuyan indicates that Qinghai black nephrite has slightly higher levels of calcium, magnesium, and silicon. Based on the regional geology of Qinghai, it is inferred that this nephrite formed through hydrothermal metasomatism of dolomitic marble, with graphite, magnetite, and other inclusions being incorporated into tremolite during the hydrothermal stage.

gems, jade, amphibole jade, nephrite, Taiwan

China has a long history of jade culture, with nephrite taking first place among the “Four Famous Jades,” giving it a broad market and widespread popularity among consumers. In this article, testing methods including polarizing microscopy, scanning electron microscopy, electron microprobe analysis, X-ray powder diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) were used to investigate the gemological and mineralogical characteristics of green nephrite from Hualien, Taiwan. Scientific, comprehensive, and systematic research is of great significance for enhancing the scientific understanding and recognition of Hualien nephrite. Observations show that the color of green nephrite ranges from light green to dark green, with relatively pure but unevenly distributed coloration. Black metallic mineral inclusions are present. The nephrite exhibits a lipidic-silky luster and is translucent to opaque. Its refractive index is 1.61, with a relative density of 2.96 to 3.02 and a hardness between 6 and 7. The primary mineral is either tremolite or actinolite, while secondary minerals include grossular, chromite, and chlorite. Electron microprobe analysis shows that the main components are MgO, SiO₂, and CaO, with small amounts of FeO and trace levels of Cr₂O₃, MnO, K₂O, Na₂O, Al₂O₃, and TiO₂. Comparison of sample data suggests that the depth of green coloration is positively correlated with FeO content, and the presence of grossular enhances color brightness. XRD results confirm that the main mineral is tremolite or actinolite. Diffraction lines of the surrounding rock match serpentine standards and indicate limited tremolite, suggesting close growth and mineral transition at the contact zone, where alteration of the host rock has occurred. FTIR analysis shows that the OH bands are separated and shifted to lower wavenumbers, likely due to Fe substituting for Mg. The absence of stretching vibrations from adsorbed water and a lack of significant absorption bands indicate that water did not occupy internal or surface space during nephrite formation. Polarizing and scanning electron microscopy reveal two microstructures: felt-like metacrystic and parallel-fibrous metacrystic, with the overall structure being schistose, while the host rock exhibits a massive texture. Comparison with nephrite from other localities shows similar characteristics, making origin identification difficult without detailed analysis of gemological features and mineral composition. The presence of locality-specific secondary minerals may aid in distinguishing between nephrites from different sources. Green nephrite from Hualien, Taiwan, is prized for its vivid color, fine texture, and unique optical effects. High-quality specimens have significant economic value, and the rare green nephrite cat’s-eye variety holds the highest value of all.

jade, jadeite, fei cui, Burma, Guatemala, geology, gems

Jadeitite is a rare rock type associated with high-pressure–low-temperature blocks within serpentinite matrix mélanges. Models of formation involve precipitation from subduction-zone aqueous fluids veining the overlying mantle wedge (P-type), or metasomatism of igneous and/or sedimentary protoliths previously emplaced into the mélange (R-type). Age determinations of mélange lithologies provide constraints on the timing of “peak metamorphism” and subsequent exhumation. The timing of jadeitite formation, particularly in the rich source of the Guatemala Suture Zone (GSZ), is a controversial subject needing further attention. Over 80 in situ zircon crystals from three jadeitites and two mica–albite rocks from the North Motagua Mélange and one phengite jadeitite from the South Motagua Mélange of the GSZ were studied for age and trace-element determination. Most of these zircons are characterized by low Th/U ratios, depleted chondrite-normalized REE patterns relative to zircons from oceanic gabbros, and contain fluid and mineral inclusions that reflect the primary mineralogy (i.e., jadeite) and context (i.e., crystallization from an aqueous fluid) of the host rock, and thus formed during jadeitite crystallization. The SHRIMP-RG and LAM-ICP-MS U–Pb dates from zircon indicate that jadeitites and mica–albite rocks from the GSZ were formed through vein precipitation at 98–80 and 154–158 Ma, respectively. These data show (a) older ages that indicate jadeitite crystallization occurred 10–30 Ma before the preserved subduction-zone peak metamorphism (e.g., exhumed eclogite), and (b) a second group of ages slightly younger than, or similar to, exhumation ages given by Ar–Ar dates from micas. Similar relationships occur at other jadeitite occurrences, such as the Syum-Keu ultramafic complex in the Polar Urals (Russia) and the serpentinite mélanges of the Río San Juan complex (Dominican Republic). The data argue for formation of jadeitite within the mantle wedge during active subduction. Thus, jadeitite provides a record of fluid introduction into the mantle wedge during subduction rather than during exhumation.

gems, spinel, spinel inclusions

Spinels contain many interesting inclusions that reveal secrets about their host rocks and sometimes also their geographic origin. The primary spinel producing countries are Burma (Myanmar), Sri Lanka, Madagascar, Tajikistan, Tanzania, and Vietnam. Spinel host rocks vary but may also have much in common, particularly those from metamorphosed dolomitic limestones (≥50% Mg replacing Ca) commonly referred to as marble. During metamorphism, calcic limestones become Magnesium rich which combines with silica to form spinel and silicates such as diopside, epidote, forsterite, garnet, zircon, etc. These same minerals may become characteristic inclusions within spinel. Other minerals that may be found in spinel include apatite, böhmite, diaspore, graphite, hercynite, högbomite, ilmenite, magnetite, various micas, rutile, sphene, and uraninite. These inclusions may be euhedral, subhedral, or corroded. The most diagnostic and commonly seen inclusions in natural spinels are octahedral negative crystals (Figure 1) that are often aligned in rows or multiple rows within their host. These negative crystals may also be seen containing white dolomite, yellow phlogopite, or black hercynite. Some inclusions or combinations of inclusions may be considered diagnostic for particular geographic origins of spinel. Transparent to white rounded apatite crystals together with attached black hexagonal crystals of ilmenite or graphite form a diagnostic inclusion scene in spinels from Burma (Myanmar). Högbomite on twinning planes is unique to spinels from Morogoro, Tanzania. Sri Lankan spinels often have an assemblage of apatite (Figure 2), diopside, phlogopite, rutile, sphene, uraninite, and zircon. Synthetic spinels are most often produced by the flame fusion process, which provides several diagnostic inclusions. Flux synthetic spinels may have typical flux inclusions but are often faceted into flawless gems, which require additional testing for conclusive identification. Inclusions in spinels are considered more useful for identifying their host than for origin determination, however, in some cases, diagnostic inclusions are evident.

gems, jade, amphibole jade, nephrite, South Korea

The Chuncheon nephrite deposit is the largest in South Korea with an estimated reserve of 300,000 tons. Located in mid-eastern South Korea, the deposit is in the form of lenses in the contact zone between dolomitic marble and amphibole schist in the upper part of a biotite schist. To date, the origin, process of formation, and age of the Chuncheon nephrite deposit remain unclear, which hinders our further understanding of the formation of nephrite. This study involved petrography, chemical analyses, and nephrite age determinations using optical mineralogy, electron microprobe-analyses (EMPA), X-ray fluorescence spectroscopy (XRF), and SHRIMP (Sensitive High-Resolution Ion MicroProbe) analyses of zircon. Petrographic studies indicated that the Chuncheon nephrite consists mainly of tremolite. The low contents of Cr (2.15–4.74 ppm) and Ni (2.95–4.90 ppm) are typical of dolomite-related nephrites and lower than in serpentine-related nephrites. The total REE contents are <18.5 ppm. The REE chondrite-normalized patterns display right-leaning LREE and almost flat HREE patterns, which are consistent with the REE model of dolomitic marble. The δ¹⁸O isotope compositions of the zircons (6.34‰–12.5‰) from the nephrite overlap those of the Chuncheon dolomite (2.4‰–18.2‰), indicating clearly that dolomite played an important role in the formation of the Chuncheon nephrite. The petrography and geochemistry suggest that it had a metasomatic origin involving the reactions dolomitic marble → diopside, diopside → tremolite, and tremolite → chlorite. The δD and δ¹⁸O isotope compositions of our specimens of nephrite (δD = −95.3‰ to −78.3‰ and −118‰ to −105‰; ¹⁸O = −9.9‰ to −7.9‰) and the hydrothermal fluids in equilibrium with the nephrite (δD = −73.6‰ to −56.6‰ and −96.3‰ to −83.3‰, 330–450 °C; δ¹⁸O = −9.41‰ to −6.38‰, 330–430 °C) suggest that the hydrothermal fluids were predominantly meteoric water. The zircons contain almost no mineral inclusions, which is quite different from zircons in other nephrite deposits. The variable SHRIMP U–Pb ages of the zircons (908.9 ± 4.8 to 459.1 ± 1.9 Ma (n = 7), 436.3 ± 16.9 to 359.5 ± 6.4 Ma (n = 5), and 249.9 ± 5.2 to 79.4 ± 1.9 Ma (n = 5)) in the Chuncheon nephrite may record the ages of a regional multi-stage metamorphism. Due to the generally low temperature of formation of the nephrite, we speculate that the ages of zircons only record the two relatively high-temperature initial stages of nephrite formation (dolomitic marble → diopside, and diopside → coarse-grained tremolite) and failed to record the age of the lowest-temperature third stage of high-quality nephrite formation (coarse-grained tremolite → fine-grained tremolite). It has generally been considered that dolomite-related nephrite forms at the contacts between granite/granodiorite and dolomitic marble, whereas serpentinite-related nephrite forms at the contacts between serpentinite and silica-rich rocks. However, the genesis of the Chuncheon nephrite deposit does not fit with either of these well-known types. Thus, we developed a new model for the Chuncheon nephrite in which heat was provided by regional multi-stage metamorphism, meteoric water dominated the nephrite-forming fluids, and dolomitic marble and biotite schist provided the material for the nephrite.

gems, jade, jadeite, fei cui, Cuba

Jadeitite (jadeite jade) from Sierra del Convento in eastern Cuba occurs in a subduction-related serpentinite-matrix mélange associated with various high-pressure tectonic blocks, including garnet-amphibolites and related anatectic trondhjemites. This jadeitite is massive and marked by rare quartz inclusions, omphacite exsolution within jadeite crystals, and replacement or infilling by omphacite. Minor minerals present include epidote, biotite, albite, phengite, titanite, rutile, zircon, and apatite. Oscillatory zoning in jadeite crystals and zircon dating indicate hydrothermal crystallization in veins within serpentinized peridotite, likely of mantle wedge origin. Al-Na-Mg-Ca-rich fluids, highly alkaline and capable of mobilizing Zr and Hf under SiO₂-subsaturated conditions, deposited jadeitite in fractures that episodically opened at depth within the subduction zone. Late-stage crystallization involved omphacite, albite, and epidote from fluids nearing SiO₂ saturation. The compositional gap observed between coexisting pyroxenes indicates a jadeite formation temperature above 500 °C. Zircon ²⁰⁶Pb/²³⁸U ages of 107.4 ± 0.5 Ma and 107.8 ± 1.1 Ma place formation at the onset of regional subduction. These ages and the elevated temperatures suggest that the jadeite-forming fluids may have originated from coeval anatectic trondhjemites crystallizing at depths around 15 kbar.

spinel, Vietnam, Luc Yen, gems

Gem spinels from a new source at Lang Chap, Vietnam, have been investigated for their gemmological, spectroscopic and chemical characteristics. The Lang Chap mining site produces spinels of orangey-red colour with inclusions and chemical composition which closely resemble similar coloured spinels from Myanmar. Experimental heat treatment at 850°C of one test sample resulted in reduction of the orange component of its colour. XRD, Raman and PL spectra indicate some disorder in the spinel structure after heating.

corundum, Burma.12, gems

The word trapiche, Spanish for “mill,” refers to the wheel that was once used to crush sugar cane. The same term is used for gems that show a characteristic six-rayed star-shaped growth zoning in cross section. This peculiar appearance, long thought to occur only in emerald, is due to bands of inclusions that radiate from the center of the crystal—the optic axis—toward the prism faces. Only in the past 20 years have other gems been discovered with a district trapiche structure, namely ruby and sapphire (K. Schmetzer et al., “Trapiche rubies,” Win-ter 1996 G&G, pp. 242–250) and tourmaline (T. Hain-schwang et al., “Trapiche tourmaline from Zambia,” Spring 2007 G&G, pp. 36–46). Generally, the trapiche ef-fect in these stones (as in emerald) is caused by dark inclu-sions. In some blue sapphires, however, it is due to strong color banding parallel to the hexagonal growth zoning. Such trapiche sapphires are known only from the Mogok region of Myanmar (K. Schmetzer, pers. comm., 2012). Recently, the Gübelin Gem Lab examined such a sap-phire, a 36.16 ct cabochon measuring 22.91 × 16.93 × 6.69 mm (figure 31). The stone showed a very pronounced blue zoning along the corners of the hexagonal growth struc-ture, while zones corresponding to the prism faces were white. In addition, many of the boundaries between the blue and the white areas displayed elongated white patches perpendicular to the growth direction of the prism faces (figure 32). Semiquantitative chemical analysis using EDXRF spec-troscopy identified both areas as corundum, with similar chemical compositions. Yet the Ti and Fe values (which cause the blue color in sapphire) as well as the Cr content were higher in the blue areas. Comparing the chemical composition of the blue portions with average concentra-tions of these elements in Burmese blue sapphires also in-dicated Myanmar as a likely origin.

gems, chrysoprase, chalchedony, quartz, Turkey, Biga–Çanakkale

The commercial quantities of gem-quality dark green chrysoprase are found as fracture fillings covered with a weathering crust in the silicified serpentinites throughout the border of a metamorphic zone in the Biga–Çanakkale region of Turkey. However, green-stained opaque quartz materials are also present in the same deposit, but these materials are common and in low demand compared to chrysoprase in terms of gemmological importance. Thus, it is necessary to distinguish these two similar materials from each other non-destructively. In addition, all chrysoprase roughs in this deposit also have alpha-quartz and moganite inclusions. Accordingly, dispersive (visible) confocal micro-Raman spectroscopy (DCμRS) allows us to distinguish clearly the chalcedonic-quartz silica phase (fibrous quartz (chalcedony)) from the crystalline-quartz silica phase (fine-grained alpha-quartz) in the case of both quartz inclusions in the chrysoprase material and the green-stained quartz material in the same deposit. This study characterizes the Biga chrysoprase (Turkey) in terms of silica building phases, chemical content, and individual Raman bands, using several destructive and non-destructive analytical techniques. The Raman spectra show that the most characteristic intensive and the widest Raman bands peaked at about 498 and 460 cm⁻¹ can be inferred to ν₂ doubly symmetric bending modes of [SiO₄/M] centers. The “M” includes some cationic substitutions of Si by Fe, Cr, Mn, As, Ni, Pb, Sb, and Zn, and K and Na as well. The second characteristic Raman band peaked at about 206 cm⁻¹ can be inferred to a single translational libration mode. The last readable Raman bands peaked at about 139 and 126 cm⁻¹ can be inferred to doubly translational libration modes as well. In addition, the weaker Raman bands peaked at about 1577, 1430, 1303, 1160, 1082, 549, 394, 352, and 259 cm⁻¹ are also present. As a result, the dispersive confocal micro-Raman spectrum of chrysoprase is directly related to its silica building phases and trace element implications.

gems, jade, nephrite, China, Xinjiang, Khotan, Hetian

The Hetian nephrite deposit in Xinjiang, Northwest China, is one of the world’s largest nephrite sources. Within this deposit, the Alamas ore body occurs as veins along the contact between Late Variscan granodiorite and Precambrian dolomitic marble. Petrographic and electron microprobe analyses indicate two potential formation pathways for nephrite: (1) direct transformation of dolomitic marble into tremolite, and (2) dolomitic marble → diopside → tremolite. Nephrite veins display color zoning from white and white-green to green toward the granodiorite contact, corresponding to increasing Cr, FeO, and TFe₂O₃ concentrations. Compared to serpentinite-related nephrite, Alamas nephrite has much lower Cr (8.95–178.7 ppm) and Ni (0.05–3.95 ppm) contents. Bulk-rock REE patterns show strong negative Eu anomalies (0.03–0.21), declining LREE, flat HREE, and low ΣREE (2.84–84.81 ppm), similar to host dolomitic marble (Eu/Eu* = 0.15–0.23, ΣREE = 8.48–11.1 ppm), suggesting a close genetic link. Fluid inclusion homogenization temperatures in tremolite yield a minimum formation temperature of 293 °C. Nephrite δ¹⁸O and δD values range from +3.2 to +6.2‰ and −83.0 to −94.7‰, respectively, with equilibrated fluid δ¹⁸O values of +3.1 to +6.1‰ at 293 °C. These isotopic and petrographic data suggest at least three fluid sources—meteoric water, magmatic water, and CO₂ from dolomite decarbonation—and support a metasomatic, dolomite-related origin for the Alamas ore body.