gems, spinel, Tanzania, Mahenge, inclusions

Describes thin exsolved “ghost” planes in Tanzania spinel.

gems, corundum, Madagascar.12, inclusions.5, treatments.6

gems, emerald, beryl, Pakistan, Swat, Mingora, inclusions, mineral inclusions, fluid inclusions, chemistry, spectra, geology

Thesis study on the emeralds of Swat, Pakistan and how their inclusions relate to their geological formation. Nice inclusion photos.

corundum, geology.11, Greenland.12, gems, Ruby

Corundum, including ruby, occurs widely in the North Atlantic Craton of southern Greenland, particularly where high-grade metamorphic lithologies of contrasting chemistry interact. This study constrains the formation conditions and fluid compositions at the Storø and Maniitsoq localities through thermodynamic modeling of mineral and mineral–fluid equilibria and solubility. Metasomatism occurred at 650–725 °C and 7 kbar, matching peak metamorphic conditions, and involved an acidic, boron-rich fluid with low fO₂ and low X(CO₂). Low aqueous Al concentrations indicate that corundum saturation resulted from residual Al enrichment rather than mobilization. A magmatic intrusion, likely the Qôrqut granite, is the probable B source, supported by U–Pb dating of rutile inclusions that link corundum formation with granite emplacement. Interaction of this oxidized, granite-derived fluid with metadunite and metapelite-hosted Fe-sulfides modified its composition, introducing SO₄ and generating a reduced, low-pH, high-XMg, high-K/Na fluid, as inferred from the corundum-bearing assemblages. These findings underscore the complex lithological interplay in high-grade metamorphic–metasomatic environments but also show that corundum formation is a geologically predictable process triggered by fluid pulses from intrusive bodies through aluminous, Si-deficient host rocks.

gems, spinel, Pakistan

Blue spinel coloured by cobalt was recently found near the Hunza Valley of northern Pakistan. Characterisation of 35 samples showed that hue variations ranging from bright blue to violet-blue to dark blue are due to variable relative concentrations of the colour-causing elements Co, Fe and Cr. Most of the spinels were semi-transparent and contained abundant internal features, including fractures, twin lamellae and growth structures, as well as distinctive iridescent geometric sectors. Solid inclusions consisted of rutile, zircon, apatite, phlogopite and calcite; fluid inclusions were rare. Samples displaying bright blue colouration typical of so-called Co-spinel had strong absorptions due to Co2+ along with subordinate Fe2+ features in their UV-Vis spectra, and contained relatively low ratios of Fe/Co (<110) and Cr/Co (<4).

gems, corundum, inclusions.5, China.12, ruby

gems, agate, quartz, chalcedony, silica minerals, micro-structure, trace elements, O-isotopes, paragenetic minerals, analytical, species

Agate—a striking variety of SiO₂ and a renowned gemstone—is typically identified as banded chalcedony. On closer inspection, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite, and/or moganite. Additionally, agates often contain significant mineral inclusions and water, present both as interstitial molecular H₂O and as silanol groups. Most agates worldwide are associated with SiO₂-rich (rhyolites, rhyodacites) and SiO₂-poor (andesites, basalts) volcanic rocks, though they may also form as hydrothermal vein types or from silica accumulation during diagenesis in sedimentary environments. The silica supply for agate formation is frequently linked to late- or post-volcanic alteration of host volcanic rocks, as indicated by associations with secondary minerals such as clays, zeolites, iron oxides/hydroxides, common pseudomorphs (e.g., after carbonates or sulfates), and the chemical composition of agates. Elements typical of the volcanic matrix (Al, Ca, Fe, Na, K) are often enriched, with unusually high levels of Ge (>90 ppm), B (>40 ppm), and U (>20 ppm) also reported. Fluid inclusion and oxygen isotope studies suggest agate forms at temperatures ranging from 20 to 230 °C. Silicic acid accumulation and condensation produce silica sols and presumably amorphous silica, serving as precursors for the characteristic agate microstructure. Crystallization generally begins with spherulitic growth of chalcedony, progressing into chalcedony fibers. High defect concentrations (oxygen and silicon vacancies, silanol groups), revealed by cathodoluminescence and electron paramagnetic resonance spectroscopy, point to rapid crystallization from an amorphous silica precursor under non-equilibrium conditions. The development of agate’s distinctive microstructure is thought to result from self-organization processes. Variations in crystallite size, porosity, silica phase, and embedded pigments ultimately lead to the iconic banding and color patterns of agate.

gems, jade, amphibole jade, nephrite, China, Russia

The green nephrite produced at the Ospinsk is the best quality green nephrite in the world. In this paper, through a variety of modern testing techniques, the gem mineralogical characteristics of the green nephrite in the Ospinsk are studied. The green nephrite in the Ospinsk’s color is mainly green and dark green, with uniform color distribution and black dot-like inclusions. The polished surface of the sample is from oily luster to weak glass luster and translucent. The relative density range is 2.98–3.00 g/cm³, the refractive index range is 1.60–1.62, and the Mohs hardness is 5.75–5.82. The main structure of green nephrite in the Ospinsk is cryptocrystalline–microscopic and unequal-grained crystal structure. The main mineral component of green nephrite in the Ospinsk is tremolite, which may contain a small amount of chromite, chlorite and so on. The main chemical components are SiO₂, MgO, CaO, and may contain a small amount of FeO, Al₂O₃, NiO, Cr₂O₃, MnO. The data obtained and calculated under the detection of electronic probes, although most of the Mg/(Mg + Fe) ratio is between 0.90–1.00, but generally concentrated between 0.90–0.92, the ratio is lower. The measured data of actinolite is around 0.89, it can be speculated that some tremolite in the sample is in the transition stage to actinolite. The ratio of Fe/(Fe + Mg) in the sample ranges from 0.08 to 0.11. The Fe/(Fe + Mg) ratio of serpentine Hetian jade is greater than 0.06. The content of Cr, Co and Ni in the samples of the LA-ICP-MS test data of the green nephrite of the Ospinsk conforms to the content range of the origin of serpentine nephrite, which corroborates the genetic types of the jasper of the Ospinsk. The total amount of rare earth elements does not change much, the content is low, and the average content is 1.498 μg/g. There is a slight negative Eu anomaly, = 0.053–0.422, Nb shows a slight loss, the value of Nb/Ta is 1.04–5.00, Laᴺ/Ybᴺ = 5.39–15.53, Laᴺ/Smᴺ < 1, Sr/Ba ratio is 1.06–2.11, the serpentinized peridot that formed green nephrite has a lower ΣREE content, LREE is weakly enriched relative to HREE, and exhibits Eu weak negative anomalie. It was found that the data detected in the infrared spectrum and Raman spectrum detected by the transmission method were consistent with the standard spectrum of amphibole minerals. The relative shift of the peak and peak position proves that the main component of the Ospinsk, the main component of the green nephrite in the Ospinsk, is in the transition to the sunburst. The green nephrite at the Ospinsk was originally produced in the ophiolite in the Eastern Sayan Mountains, and the green curtain-hornblende transition facies in the metamorphic rocks at the bottom of the ophiolite. Under the condition of low temperature hydrothermal fluid, the ultrabasic rock of forsterite and the low temperature hydrothermal fluid interact to form serpentine. Serpentine is then contacted with surrounding rocks rich in silicon and calcium to form tremolite, which is the main component of green nephrite in the Ospinsk.

gems, corundum, ruby, Tajikistan.12, Snezhnoe deposit

The Snezhnoe ruby deposit is located in the Muzkol–Rangkul anticlinorium within the Cimmerian zone of the Central Pamir. On the local scale, the deposit occurs on discrete relict bedding planes of calcitic marbles belonging to the Sarydzhilgin suite. Four ruby-bearing mineral assemblages are present within the main parts of the deposit: (1) scapolite + phlogopite + muscovite + margarite; (2) plagioclase + muscovite + margarite; (3) muscovite + phlogopite + margarite; (4) calcite. The ruby + calcite association is the most economically important, whereas the association of plagioclase + scapolite + phlogopite + muscovite is typical for the ruby-free parts of the deposit. Mica group minerals with a distinctive green color due to enhanced Cr and V concentrations are the main prospecting indicators for the ruby mineralization. The oxygen isotopic composition of the rubies is +15.3‰, a common value for crustal metamorphic and sedimentary rocks. The ratios of indicative trace elements in the rubies are Ga/Mg < 8.2, Fe/Mg < 51.2, Cr/Ga > 6.9 and Fe/Ti < 31.6. These values are characteristic for metamorphic corundum. The bulk ruby-bearing rocks have an initial 87Sr/86Sr ratio of ~0.70791 and εNd of ~−9.6, also pointing to the crustal origin of the deposit in agreement with the geological data. Ancient Al-enriched sediments are suggested to be a possible protolith for the ruby-bearing rocks. The temperature of the metamorphic processes was estimated at 760 ± 30°C using Zr-in-rutile geothermometry. Raman mapping of rutile inclusions trapped within the ruby crystal indicates that the minimum pressure of mineralization was about one kilobar. The age determined by the Rb–Sr thermal ionization mass spectrometry of phlogopite, plagioclase and bulk rock is 23 ± 1.6 Ma, corresponding to the timing of relaxation after peak metamorphism during the Alpine–Himalayan Orogeny.

gems, beryl, emerald, Davdar, China

Brief article on the inclusions in Davdar, China emeralds.