gems, jade, amphibole jade, nephrite, China

The No. 7 nephrite deposit is located in Qiemo, Xinjiang, northwest China, within the Kunlun–Altyn nephrite belt along the southern margin of the Tarim Basin. Field observations and hand specimen analysis reveal distinct mineralization zoning: a magnesian marble belt, a serpentine-altered marble and gray-white nephrite contact belt, a diopside-altered marble belt, a gray nephrite belt, and a granite intrusion belt. The gray, gray-white, and white nephrite is relatively pure, composed mainly of tremolite, with minimal inclusions aside from scattered apatite, chlorite, and mica. It is inferred that these nephrites formed from hot hydrothermal fluids during a short-term rapid cooling phase of metasomatism. Major element analysis suggests that color zoning is related to Fe and Mn content, with increasing concentrations corresponding to a shift from white to green. Trace and rare earth element levels are low overall, with enrichment in LREE and relative depletion in Nb, Sr, and Hf, while U, La, P, and Ti are enriched. A negative Eu anomaly (0.18–0.51) suggests plagioclase separation during crystallization. Polarizing microscope and BSE imagery show high-quality nephrite with a feather-like micro-interlaced texture. Textural replacement relationships—calcite by diopside and tremolite, and later fine-grained tremolite replacing coarse-grained diopside and tremolite—indicate the nephrite formed through multiple stages of magmatic hydrothermal metasomatism.

gems, corundum, History.1, Sri Lanka.12, basalt sapphire

The gemmological analysis of a Roman intaglio engraved with a hippocamp (winged ‘sea-horse’) reveals that it was carved from a sapphire of basaltic origin. Its bluish grey appearance is due to Rayleigh scattering by sub-microscopic inclusions and is not related to an intervalence charge transfer process. In light of historically documented extensive trade relations between ancient Rome and Ethiopia (the kingdom of Aksum), we hypothesise that the recently documented basalt-related sapphire deposits in northern Ethiopia are a possible source of raw material for this Roman intaglio (as well as other basaltic sapphires used in Roman times), in addition to previously held views that ancient sapphires originate from deposits in Sri Lanka, France and perhaps South East Asia. This rare opportunity to characterise in detail one of the very few engraved sapphires from the Roman period permits a better understanding of gem materials used in classical antiquity.

gems, corundum, Burma.12, Mogok, ruby, sapphire

Ruby in diverse geological settings leaves petrogenetic clues, in its zoning, inclusions, trace elements and oxygen isotope values. Rock-hosted and isolated crystals are compared from Myanmar, SE Asia, and New South Wales, East Australia. Myanmar ruby typifies metasomatized and metamorphic settings, while East Australian ruby xenocrysts are derived from basalts that tapped underlying fold belts. The respective suites include homogeneous ruby; bi-colored inner (violet blue) and outer (red) zoned ruby; ruby-sapphirine-spinel composites; pink to red grains and multi-zoned crystals of red-pink-white-violet (core to rim). Ruby ages were determined by using U-Pb isotopes in titanite inclusions (Thurein Taung; 32.4 Ma) and zircon inclusions (Mong Hsu; 23.9 Ma) and basalt dating in NSW, >60–40 Ma. Trace element oxide plots suggest marble sources for Thurein Taung and Mong Hsu ruby and ultramafic-mafic sources for Mong Hsu (dark cores). NSW rubies suggest metasomatic (Barrington Tops), ultramafic to mafic (Macquarie River) and metasomatic-magmatic (New England) sources. A previous study showed that Cr/Ga vs. Fe/(V + Ti) plots separate Mong Hsu ruby from other ruby fields, but did not test Mogok ruby. Thurein Taung ruby, tested here, plotted separately to Mong Hsu ruby. A Fe-Ga/Mg diagram splits ruby suites into various fields (Ga/Mg < 3), except for magmatic input into rare Mogok and Australian ruby (Ga/Mg > 6). The diverse results emphasize ruby’s potential for geographic typing.

gems, corundum, sapphire, inclusions.5

A phantom sapphire crystal within a host sapphire crystal was recently examined at GIA’s Carlsbad laboratory. The inclusion showed euhedral morphology as one half of a partially tapered bipyramid. The visible boundaries of the included crystal were composed of unaltered fingerprints with geometric patterns and fine linear rows. The crystal’s surface also showed moderate-order birefringence colors; these represented the shared interface between the host’s crystal lattice and the inclusion’s crystal lattice. External local fingerprints also terminated around the crystal’s end point. Negative crystals within the inclusion showed tabular hexagonal habit. When the host sapphire was exposed to temperatures below the freezing point of water, a CO₂ bubble was seen within the largest negative crystal. A sapphire with this inclusion combination is rarely seen.

gems, corundum, inclusions.5

GIA’s Hong Kong laboratory recently examined a remarkable sapphire inclusion inside a Burmese star sapphire. Possessing the same refractive index as its host, the sapphire guest displays extremely low relief and is nearly invisible under transmitted light. Nevertheless, its outline became distinctive under cross-polarized illumination, showing strong interference colors (figure 11, left). Closer examination using fiber-optic illumination revealed a group of oriented rutile “silk” in this sapphire inclusion (figure 11, right). Although exsolved rutile needles are a common inclusion in Burmese sapphire, it is interesting to see two sets of dense silk exsolved from both the star sapphire host and a protogenetic sapphire inclusion, each aligned with the basal plane of its own host.

gems, corundum, color.4, spectroscopy, FTIR, infrared spectra

The 3161 cm⁻¹ mid-infrared (IR) spectral feature (figure 1) is an important tool in the identification of unheated sapphires, especially in material from low-iron metamorphic environments such as Sri Lanka. This feature is a series of bands, composed of a strong peak at ~3161 cm⁻¹ and smaller side bands at ~3075, 3240, and 3355 cm⁻¹. A few researchers have attributed these features to OH groups involved in charge compensation with Si⁴⁺, while some have assigned them to structurally bonded OH, associated with Mg²⁺. The 3161 cm⁻¹ series is more commonly observed in natural-colour yellow sapphires than any other colour of corundum; occasionally it is encountered in blues and pinks. These authors, however, have encountered a strong feature at ~3161 cm⁻¹ in a few specimens of yellow synthetic sapphire grown by the flame-fusion (Verneuil) process. Their synthetic origin was determined on the basis of inclusion study, which revealed the presence of clouds of minute gas bubbles, along with some bomb-shaped gas bubbles, typically associated with corundum or spinel grown by the flame-fusion process. Milky zones of fine pinpoints and a “plato” effect were also present. The 3161 cm⁻¹ mid-IR feature in these synthetic yellow sapphires displayed side bands at approximately 3220 and 3277 cm⁻¹, as opposed to 3240 and 3355 cm⁻¹ in natural sapphires.

gems, quartz, amethyst, volcanic rock, hydrothermal alteration, Greece, gemstones

Epithermally altered volcanic rocks in Greece host amethyst-bearing veins in association with various silicates, carbonates, oxides and sulfides. Host rocks are Oligocene to Pleistocene calc-alkaline to shoshonitic lavas and pyroclastics of intermediate to acidic composition. The veins are integral parts of high to intermediate sulfidation epithermal mineralized centers in northern Greece (e.g., Kassiteres–Sapes, Kirki, Kornofolia/Soufli, Lesvos Island) and on Milos Island. Colloform–crustiform banding with alternations of amethyst, chalcedony and/or carbonates is a common characteristic of the studied amethyst-bearing veins. Hydrothermal alteration around the quartz veins includes sericitic, K-feldspar (adularia), propylitic and zeolitic types. Precipitation of amethyst took place from near-neutral to alkaline fluids, as indicated by the presence of various amounts of gangue adularia, calcite, zeolites, chlorite and smectite. Fluid inclusion data suggest that the studied amethyst was formed by hydrothermal fluids with relatively low temperatures (~200–250°C) and low to moderate salinity (1–8 wt % NaCl equiv). A fluid cooling gradually from the external to the inner parts of the veins, possibly with subsequent boiling in an open system, is considered for the amethysts of Silver Hill in Sapes and Kassiteres. Amethysts from Kornofolia, Megala Therma, Kalogries and Chondro Vouno were formed by mixing of moderately saline hydrothermal fluids with low-salinity fluids at relatively lower temperatures indicating the presence of dilution processes and probably boiling in an open system. Stable isotope data point to mixing between magmatic and marine (and/or meteoric) waters and are consistent with the oxidizing conditions required for amethyst formation.

gems, corundum, geology.11, Thailand.12, USA.12, Montana.12, Yogo, Chanthaburi-Trat

A wide number of genetic models have been proposed for volcanically transported ruby and sapphire deposits around the world. In this contribution we compare the trace element chemistry, mineral and melt inclusions, and oxygen isotope ratios in blue to reddish-violet sapphires from Yogo Gulch, Montana, U.S.A., with rubies from the Chantaburi-Trat region of Thailand and the Pailin region of Cambodia. The similarities between Thai/Cambodian rubies and Yogo sapphires suggest a common origin for gem corundum from both deposits. Specifically, we advance a model whereby sapphires and rubies formed through a peritectic melting reaction when the lamprophyre or basalts that transported the gem corundum to the surface partially melted Al-rich lower crustal rocks. Furthermore, we suggest the protolith of the rubies and sapphires was an anorthosite or, in the case of Thai/Cambodian rubies, an anorthosite subjected to higher pressures and converted into a garnet-clinopyroxenite. In this model the rubies and sapphires are rightfully considered to be xenocrysts in their host basalts or lamprophyre; however, in this scenario they are not “accidental” xenocrysts but their formation is intimately and directly linked to the magmas that transported them to the surface. The similarities in these gem corundum deposits suggests that the partial melting, non-accidental xenocryst model may be more wide-reaching and globally important than previously realized. Importantly, in both cases the gem corundum has an ostensibly “metamorphic” trace element signature, whereas the presence of silicate melt (or magma) inclusions shows they ought to be considered to be “magmatic” rubies and sapphires. This discrepancy suggests that existing trace element discriminant diagrams intended to separate “metamorphic” from “magmatic” gem corundum ought to be used with caution.

gems, jade, amphibole jade, nephrite, China

Based on the test methods such as petrography, electron probe microanalysis, cathode luminescence, X-ray powder diffraction, laser Raman spectroscopy, infrared spectroscopy, UV-visible spectroscopy and so on, the mineral composition, chemical component, spectroscopy characteristics were systematically studied in this paper and the implications for graphite inclusions were also discussed. The Sangpiyu tremolite jade shows various colours including bluish white, yellow green, deep green and dark green, although pure white variety has not yet been found. The Sangpiyu tremolite jade contains abundant graphite inclusions, mostly showing striated distribution pattern in dark green and yellow green varieties and belts of graphite in the deep green types. In dark green sample, the black belts of graphite are rather developed. Polarizing microscope reveals that the main mineral composition of the Sangpiyu tremolite jade is tremolite, with minor calcite, dolomite, zoisite, titanite, allanite, apatite and graphite. The textures of Sangpiyu tremolite jade are felt-like interweave, microfibril crystal, microscopic radial (broom) and micro-texture of crystal blade etc. The mineral grains of Sangpiyu tremolite are abundant, this suggests that tremolite jade formation process can be divided into fourth phases: platy, columnar or lamellar, fibrous and subtle fibrous. The EMPA shows that tremolite jade in Sangpiyu is predominately composed of tremolite with minor calcite, allanite, chlorite, apatite, chromite and graphite. Whereas, the major minerals of the wall rocks are plagioclase. The main chemical components of the Sangpiyu tremolite in Liaoning Province are SiO₂, MgO and CaO. Among them, the content of SiO₂ is generally more than 60%, and the content of MgO and CaO in the whole samples is slightly lower. XRD reveals that Sangpiyu tremolite is predominately composed of tremolite with minor calcite, serpentine and graphite. The graphite layers in dark green samples show a relatively constant d(002) = 0.33562, and suggest that these were formed at relatively high temperature metamorphic conditions. Based on the information of U and Rh%, it can further prove that the graphite inclusions in this kind of samples crystallize are much better. Raman spectroscopy of graphite inclusions shows that the D₁/G intensity ratio ranges from 0.78 to 0.88 in deep green samples and from 0.05 to 0.23 in dark green samples. The ranges of D₁/(D₁ + G) integral area ratio for these types are from 0.0548 to 0.3037 and 0.5528 to 0.7355 respectively. The formation temperature of graphite inclusions in the dark green tremolite jade is computed as 549.8 °C, whereas that for the deep green sample is about 343.2 °C. Our results suggest that the jade formation occurred in a multi-stage process through the action of hydrothermal fluids and metamorphism possibly in a subduction-related setting at moderate to high temperatures.

gems, jade, amphibole jade, nephrite, China

Hetian jade, with its rich cultural heritage, has seen changes in mining technology and geographical sources due to economic development. Apart from Xinjiang, known sources include Golmud in Qinghai, Xiuyan in Liaoning, Luodian in Guizhou, Hualien in Taiwan, and Luoyang in Henan. In 2016, a suspected nephrite deposit was discovered in TangGuanPu, XiangHuaLing area of Linwu, Hunan. The TangGuanPu area is situated at the contact zone of granite and carbonate rock in Hunan. Initially known for tin polymetallic deposits, nephrite is found as an accessory mineral. The nephrite deposits are primarily of the contact metasomatic type, resulting from contact metasomatism between granite and calcium-magnesium carbonate. Silicon-card lithified diopside, garnet, and later hydrous silicate minerals like tremolite and actinolite were formed through metasomatism. The nephrite veins in the area are about 0.5 to 1.5 meters wide, often exhibiting blue and green colors. Inclusions of quartz and metal minerals like arsenopyrite and pyrrhotite are common, affecting the transparency and appearance of the nephrite. The second mining area, being a tin polymetallic deposit, has additional minerals such as magnetic arsenopyrite and pyrite. Analyzing electron probe and X-ray fluorescence spectrum data revealed high iron content in the nephrite from the TangGuanPu area, averaging 4.366%. Other impurity elements include potassium, manganese, sodium, phosphorus, titanium, nickel, sulfur, chromium, and aluminum, contributing to uneven coloration and the presence of white veins and dark metal particles. The Mg/(Mg + Fe²⁺) ratio of 0.817 distinguishes the nephrite from Fragrant Hill as Yang nephrite, highlighting a significant difference from Xinjiang and Qinghai nephrite jade.