Asia, Ruby, Marble, Evaporite, Metamorphism, Geochemistry, Stable isotopes, Genetic model, corundum, geology.11, gems

Marble-hosted ruby deposits represent the most important source of colored gemstones from Central and Southeast Asia. These deposits are located in the Himalayan mountain belt, which formed during the Tertiary collision of the Indian plate moving northward into the Eurasian plate. Spatially associated with granitoid intrusions and hosted in platform carbonates that underwent high-grade metamorphism, these ruby deposits occur near major tectonic features formed during Himalayan orogenesis—either directly in suture zones in the Himalayas or in shear zones that facilitated the extrusion of the Indochina block following the collision in Southeast Asia. Ar–Ar dating of micas syngenetic with ruby and U–Pb dating of zircon inclusions confirm that ruby formation occurred during Himalayan orogenesis, reflecting extensional tectonics active from Afghanistan to Vietnam between the Oligocene and Pliocene. Petrographic analysis shows that ruby-bearing marbles formed under amphibolite facies conditions (T = 610–790 °C, P ~6 kbar), while fluid inclusion studies define ruby crystallization conditions during the retrograde metamorphic path (620–670 °C, 2.6–3.3 kbar) at Jegdalek, Hunza, and northern Vietnam. Whole-rock analyses of non-ruby-bearing marbles reveal sufficient aluminum and chromium to account for all observed ruby. Stable isotope data (C, O) confirm a closed metamorphic fluid system, uninfluenced by external sources. Graphite’s carbon isotopic composition indicates an organic origin, with isotopic exchange with carbonate minerals during metamorphism; ruby oxygen isotopes were buffered by CO₂ released from devolatilizing carbonates, and mica hydrogen isotopes support a metamorphic water source. The C–O–H isotopic data and fluid inclusions containing CO₂–H₂S–COS–S₈–AlO(OH) suggest ruby formed at ~3 kbar and 620–670 °C through thermal reduction of evaporite by organic matter during high-temperature, medium-pressure metamorphism of platform carbonates during the India–Asia collision. These carbonates were enriched in detrital Al- and Cr-bearing clays and organic matter. Ruby crystallized during retrogression, primarily via destabilization of muscovite or spinel, in a CO₂-rich metamorphic fluid system enhanced by fluorine, chlorine, and boron from molten salts (NaCl, KCl, CaSO₄). The presence of evaporites was critical, as molten salts mobilized aluminum and transition metals in situ, enabling ruby crystallization.

gems, opal, Iran, blue opal

Gem-grade blue-green opal, traditionally sourced from Peru, has recently been discovered in Iran, approximately 110 km northwest of Shahr-e Babak in Kerman Province. According to Makhmout Douman (Arzawa Mineralogical Inc., New York), mining at this new locality had nearly ceased by October 2007 due to groundwater intrusion, and few polished stones have been produced because of the fractured and thin nature of the rough. Mr. Douman loaned GIA two cabochons (2.40 and 2.59 ct) and one rough piece for analysis. The cabochons showed bluish green and green-blue color, were translucent, and had refractive indices of 1.45 and 1.47, respectively, with specific gravities of 2.06 and 2.00. No reaction was observed under a Chelsea filter or UV light, and a 600 nm cutoff was seen in the spectroscope. Microscopic examination revealed milky white clouds and, in one sample, a moss-like inclusion. Although the 1.47 RI is slightly above the typical opal range (1.44–1.46), Raman spectra matched those of standard opal. Near-IR spectroscopy confirmed hydroxyl-related features characteristic of opal, while EDXRF analysis detected major Si, minor Cu, and trace Fe. UV-Vis-NIR spectroscopy indicated Cu²⁺ absorptions between 527–1176 nm, confirming copper as the chromophore responsible for the opal’s blue-green coloration.

gems, corundum, synthetic corundum.7, Kashan, treatments.6

From G&G: In March 2006, a 10.12 ct transparent purplish red oval mixed-cut gemstone was submitted to the Dubai Gemstone Laboratory for identification, initially reported by the client as a natural ruby from Myanmar. Standard gemological testing confirmed properties consistent with ruby, but strong chalky blue fluorescence under short-wave UV radiation suggested heat treatment. Microscopic examination revealed inclusions resembling those in flux-assisted heated natural rubies; however, further analysis identified various forms of flux residue, characteristic features of Kashan flux-grown synthetic rubies, along with white high-relief parallel rods, fingerprint-like structures, rain-like comet formations, and discoid features with paint-splash inclusions. Additional signs of heat treatment included dissolved white flux-filled negative crystals with associated discoid fissures. UV-Vis absorption spectroscopy exhibited broad bands typical of ruby color, but also showed UV transmission at 305 nm stronger than visible-region transmission at ~475 nm, indicative of synthetic origin. This pattern was previously documented in heat-treated Kashan synthetic rubies and linked to higher titanium content. Infrared spectroscopy revealed no hydroxyl-related peaks, consistent with flux-grown synthetics, while EDXRF analysis detected trace elements distinguishing Kashan flux-grown rubies, with slightly higher titanium and traces of vanadium and gallium. Comparison with a reference collection confirmed a match to Kashan specimens. Since the 1990s, heat-treated synthetic rubies have posed increasing identification challenges, necessitating advanced testing for accurate distinction.

gems, corundum, inclusions.5, beryl, emerald, synthetic beryl, emerald, pezzottaite, spinel

Nail-head spicules are inclusions that have traditionally been associated with hydrothermal synthetic quartz and emerald. They are caused primarily by rapid growth conditions and disturbances during crystallization of their host. However, these or similar-looking inclusions have also been found in natural gems. The authors examined a natural emerald and blue sapphire showing true nail-head spicules, and a yellow sapphire, spinel, diamond, and rock crystal quartz with inclusions that strongly resembled them. Nail-head spicules remain a notable feature of rapid and disturbed growth, but they do not confirm a stone’s natural or synthetic origin without further examination.

corundum, inclusions.5, gems

This study presents noble gaseous data of the corundum megacrysts from the Cenozoic basalts in Changle, Shandong Province, eastern China. It is known that no noble gaseous data of corundum megacryst have been documented before. The ³He/⁴He ratios (1.13–7.37 Ra) of the corundums from Changle vary from atmosphere to MORB values; the ²⁰Ne/²²Ne (9.67–10.75) and ²¹Ne/²²Ne (0.0280–0.0372) data define two linear trends on Ne three-isotope diagram, respectively, along the MFL and the correlation line between atmosphere and MORB; the ³⁸Ar/³⁶Ar (0.177–0.194) ratios, the ⁴⁰Ar/³⁶Ar (280.9–404.2) ratios and the ¹²⁸–¹³⁶Xe/¹³²Xe ratio with obvious ¹²⁹Xe excess are generally higher than atmospheric component, but the ⁴⁰Ar/³⁶Ar ratios are much closer to atmospheric ratio. The isotopic compositions of noble gases (particularly for He and Ar) of the corundums are similar to those of pyroxene, anorthoclase megacrysts, and mantle-derived xenoliths from this area, and those of mantle-derived xenoliths from several areas in eastern China. Therefore, the noble gases trapped in the corundums probably are from mantle source, representing a “mixed fluid” produced by the interaction between the lithospheric mantle and fluids releasing from the convective plate. Both the noble gas isotopic compositions and the oxygen isotopic compositions of the solid corundums are not the characteristics of crustal source. These suggest that the corundums crystallized from mantle-derived magmas with minimal crustal contamination.

gems, jade, pyroxene jade, fei cui, Burma, Lawsonite, Eclogite, geology, Guatemala

Lawsonite eclogites preserve a record of very-low-temperature conditions in subduction zones. All occur at active margin settings, typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite-matrix mélange. Peak lawsonite-eclogite facies mineral assemblages (garnet+omphacite+lawsonite+rutile) typically occur in prograde-zoned garnet porphyroblasts. Their matrix is commonly overprinted by higher-temperature epidote-bearing assemblages; greenschist- or amphibolite-facies conditions erase former lawsonite-eclogite relics. Various pseudomorphs after lawsonite occur, particularly in some blueschist/eclogite transitional facies rocks. Coesite-bearing lawsonite-eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low-temperature ultrahigh-pressure eclogites are known. Using inclusion assemblages in garnet, lawsonite eclogites can be classified into two types: L-type, such as those from Guatemala and British Columbia, contain garnet porphyroblasts that grew only within the lawsonite stability field and E-type, such as from the Dominican Republic, record maximum temperature in the epidote-stability field. Formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation. The earliest occurrences of lawsonite-eclogite facies mineral assemblages are Early Paleozoic in Spitsbergen and the New England fold belt of Australia; this suggests that since the Phanerozoic, secular cooling of Earth and subduction-zone thermal structures evolved the necessary high pressure/temperature conditions. Buoyancy of serpentinite and oblique convergence with a major strike-slip component may facilitate the exhumation of lawsonite eclogites from mantle depths.

gems, beryl, emerald, Kazakhstan, Delbegetey, inclusions, gemology, provenance, origin determination, fluid inclusions, oxygen isotopes

The aim of this study is to provide the first detailed mineralogical and fluid-inclusion description of emeralds from the Delbegetey deposit (Kazakhstan). The characteristic features of Delbegetey emeralds are established: they show dissolution figures on crystal faces, a bluish colour with distinct colour zoning; their refractive indices are ω = 1.566–1.570 and ε = 1.558–1.562, while the specific gravity is 2.65 ± 0.005—relatively low for natural emeralds; they contain only trace amounts of common impurities (Fe, Mg, Na, etc.) and are coloured by Cr and V. Fluid inclusions of all types are strongly vapour-rich, and primary inclusions homogenize to liquid at 395–420 °C. The lattice oxygen-isotope composition (δ¹⁸O SMOW = 11.3 ‰) places the deposit within the range typical of other granite-related emerald occurrences. Emerald crystallization occurred in a low-density (0.40–0.55 g cm⁻³) aqueous fluid composed of (mol %) 75.6–97.4 H₂O, 0.0–18.4 CO₂, 0.0–0.9 CH₄, with a salinity of 4.06–9.65 wt % NaCl equiv. Isochore calculations indicate formation pressures of 570–1240 bar.

gems, olivine, peridot, chrysolite, Russia, Kola Peninsula

Gem-quality chrysolite (peridot) from a phlogopite deposit related to the Kovdor ultrabasic-alkaline massif in the Kola Peninsula, Russia, was studied using a variety of techniques (optical mineralogical microscopy, chemical, Mössbauer spectroscopy, and photoluminescence) to determine its chemical composition, the Fe²⁺/Fe³⁺ ratio, refraction indexes, density, as well as to examine inclusions in it. Much attention was devoted to the microprobe identification of crystalline inclusions in the host chrysolite (apatite, tetraferriphlogopite, amphibole, and magnetite), its exsolution products (diopside and magnetite), and the daughter phases of melt inclusions in this mineral (which were subdivided into primary and secondary genetic types). The daughter phases of these melt inclusions are silicates (forsterite, diopside, tetraferriphlogopite, clinohumite, and serpentine), various carbonates (Ca-dominated carbonates are characteristic of the primary inclusions, whereas Mg-rich carbonates were found only in the secondary inclusions), magnetite, djerfisherite (alkali sulfide), and Ba-Sr-REE carbonates. The presence of melt inclusions testifies to a magmatic genesis of the gem, and the simultaneous occurrence of these inclusions with crystalline inclusions can be used as an additional identification feature of gem chrysolite from the Kovdor Massif.

corundum, geology.11, gems

Ruby deposits hosted by marbles are distributed along the Himalayan mountain fold belt that formed during the Tertiary collision of the Indian plate with Asia, as well as in the Indochina crustal block extruded along shear zones during this collision. These metamorphic deposits are found in Tajikistan, Afghanistan, Pakistan, Nepal, Myanmar, and Vietnam. This study was undertaken to relate the age of their formation to the geologic events associated with the continental collision. Ruby itself is not suitable for age dating, but inclusions in ruby, and certain associated minerals, can both be dated by certain techniques.. Ar-Ar age dates of between 25 and 4.6 Ma document Oligocene-Miocene cooling ages for the ruby-bearing metamorphic belts, which in turn represent minimum ages for ruby formation. These ages are in agreement with published data on the tectonic-metamorphic history of the marbles hosting the rubies. U-Pb age dates of 54–36 Ma for zircon inclusions give a maximum Eocene age for ruby mineralization in Vietnam. The specific time of formation of each ruby deposit depended on its location within the fold belt.

corundum, Australia.12, gems

Corundum xenocrysts from alkaline basalt fields differ in characteristics and hence lithospheric origins. Trace element, fluid/melt inclusion, and oxygen isotope studies on two eastern Australian corundum deposits are compared to consider their origins. Sapphires from Weldborough, NE Tasmania, are magmatic (high-Ga, av. 200 ppm) and dominated by Fe (av. 3300 ppm) and variable Ti (av. 400 ppm) as chromophores. They contain Cl, Fe, Ga, Ti, and CO₂-rich fluid inclusions and give δ¹⁸O values (5.1–6.2‰) within the mantle range. Geochronology on companion zircons suggests several sources (from 290 Ma to 47 Ma) were disrupted by basaltic melts (47 ± 0.6 Ma). Gem corundums from Barrington, New South Wales, also include magmatic sapphires (Ga av. 170 ppm; δ¹⁸O 4.6–5.8‰), but with more Fe (av. 9000 ppm) and less Ti (av. 300 ppm) as chromophores. Zircon dating suggests that gem formation preceded and was overlapped by Cenozoic basaltic melt generation (59–4 Ma). In contrast, a metamorphic sapphire–ruby suite (low-Ga, av. 30 ppm) here incorporates greater Cr into the chromophores (up to 2250 ppm). Fluid inclusions are CO₂-poor, but melt inclusions suggest some alkaline melt interaction. The δ¹⁸O values (5.1–6.2‰) overlap magmatic sapphire values. Interactions at contact zones (T = 780–940 °C) between earlier Permian ultramafic bodies and later alkaline fluid activity may explain the formation of rubies.