gems, jade, fei cui, pyroxene jade, geology, Guatemala, Montagua valley, jade formation

Jadeitites and albitites from the Motagua Valley, Guatemala, are high-pressure–low-temperature metasomatic rocks that occur as tectonic inclusions in a serpentinite-matrix mélange. Metasomatism was driven by a fluid with a δ¹⁸Oₕ₂ₒ value of 6‰ and a δDₕ₂ₒ value that is high compared to metamorphic fluids from other high-pressure–low-temperature settings of similar metamorphic grade. It is inferred that the fluid was originally seawater, entrained during subduction either as mineral-bound H₂O or as free pore water. This fluid facilitated serpentinization reactions in ultramafic rocks, potentially leading to deuterium enrichment of the H₂O, before forming the jadeitites and albitites at an estimated depth of 29 ± 11 km. The isotopic signatures and fluid-inclusion data show similarities to rodingites, Ca-rich metasomatic rocks commonly found in serpentinite-bearing environments. These findings suggest that serpentinization, whether in subduction zones or at oceanic spreading ridges, may yield residual fluids that are rich in H₂O, contain 1–8 wt% equivalent NaCl, and display elevated δD values—potentially seawater-like in character.

gems, inclusions, beryl, emerald

Mineral inclusions in emeralds from eleven gem-mining regions and a hydrothermally grown synthetic emerald were chemically analysed with the electron microprobe JEOL JXA-8600. Emerald occurrences in Australia, Brazil, Mozambique, Russia, Tanzania (Lake Manyara) and Zambia belong to 'schist type' deposits. The inclusions comprise micas, talc, pyrophyllite, chlorite, wollastonite, chromite, calcite, dolomite, Fe-oxide, apatite, quartz, pyrite, fluorite and plagioclase. The geological settings for Colombian, Nigerian and Sumbawanga (Tanzania) deposits resulted in related assemblages of solid inclusions in these emeralds (for example, chlorine-rich Al- glauconite, sphalerite, titanium-rich mica, anhydrite and beryllium minerals).

corundum, sources.12, Tanzania.12, East Africa, Tunuru, Songea, gems

Gem corundum with a bright pink to purplish red hue is recovered from river gravels in the Tunduru–Songea area of southern Tanzania, and sold in Bangkok. Most, but not all, of these rubies appear to have been heat treated prior to sale. The rubies have refractive indices of nε = 1.760–1.765 and nω = 1.768–1.773 (yielding a birefringence of 0.008–0.009) and specific gravity values of 3.95–4.04. The characteristic inclusions found in both the obviously heat-treated and minimally heat-treated stones are described.

corundum, History.1, gems

For their aesthetic and symbolic value, gemstones have always been sought out and used by humans. The Mesopotamian tablets, the oldest written records, frequently mention gemstones: lapis lazuli, carnelian… However, due to their rarity, the hard precious stones of Antiquity (diamond, ruby, emerald, sapphire) have been less studied. The examination of the Parthian statuette preserved in the Louvre Museum, an object inlaid with rubies, provided an opportunity for the Laboratory of Research of the Museums of France (LRMF) to implement ion beam analysis methods for in-depth research in the field of historical gemmology. The archaeological piece at the core of this study was discovered in Mesopotamia. Based on typology, it is attributed to the Parthian period and would thus date to the 3rd century BCE. This remarkable object represents the goddess Ishtar, associated with Aphrodite. The statuette is made of alabaster; her eyes and navel are inlaid with a red stone initially thought to be glass (fig. 1). In 1995, during the dossier-exhibition Precious Stones of the Ancient East, from the Sumerians to the Sasanians organized by François Talion at the Louvre Museum, Annie Caubet, curator in charge of the Department of Near Eastern Antiquities, proposed that we study this statuette. The objectives were, on the one hand, to confirm the suggestion of S. Van Roy², who believed these cabochons to be rubies, and on the other hand, to attempt to determine the geographic origin of these gems. The study of the rubies on the Parthian statuette from the Louvre Museum combines optical observation of their inclusions with their geochemical characterization using the PIXE method (with the help of AGLAE). It first confirmed that the red inlays of the eyes and navel are indeed natural rubies. Furthermore, it provided an opportunity to build a compositional database for major ruby deposits. Finally, thanks to statistical analysis, we can propose Burma as the most probable origin of the statuette’s rubies, though Sri Lanka cannot be ruled out. These results, along with recent improvements to the AGLAE accelerator, pave the way for a complementary study to strengthen the attribution and attempt to decide between a Burmese or Sri Lankan origin for these rubies. Indeed, the new extracted beam from AGLAE now makes it possible to focus the beam on an area just 0.03 mm in diameter. This tool is well suited for characterizing the mineral inclusions observed in our study (rutile, zircons, phlogopite…), a key factor for authenticating the paragenesis and, consequently, for tracing the provenance of the gems. These gemstones are evidence of exchange or trade across several thousand kilometers between Mesopotamia and Southeast Asia in the 3rd century BCE. To our knowledge, this is the earliest known use of ruby in this region. This representation of Ishtar is remarkable because it combines influences from three civilizations: Mesopotamia through the deity represented, Greece through its Hellenistic style, and Asia through both the placement (eyes and navel) and the nature of the precious stones.

gems, beryl, emerald, Brazil, inclusions

Emerald crystals from the Campos Verdes mining district have been analysed by inductively coupled plasma mass spectrometry (ICP-MS), Fourier transform infrared spectroscopy and Mössbauer 2+ 3+ spectroscopy. These gems contain measurable amounts of Fe , Fe , Cr, V, Sc, Ni and Zn; large-ion lithophile element (LILE, Ba, Sr, Rb, Pb) and high field strength element (HFSE, Zr, Y, Ti) contents are near or below the limit of detection. This chemical signature reflects the metasomatic reactions between the mineralizing fluids and the ultrabasic host-rocks of this gem deposit.

Chemistry and Materials Science, corundum, geology.11, gems

Corundums from basalt fields, particularly in Australia and Asia, include a dominant blue-green-yellow zoned magmatic suite (BGY suite) and subsidiary vari-coloured metamorphic suites. The BGY corundums have distinctive trace element signatures, with up to 0.04 wt% Ga₂O₃ and low Cr/Ga and Ti/Ga ratios (<1). Different melt origins for the BGY suite are examined based on inclusion and intergrowth mineralogy, petrologic associations, and tectonic setting. Over 100 analysed primary mineral inclusions include typical feldspars, zircon, Nb–Ta oxides, hercynite–magnetite, gahnospinel, rutile–ilmenite solid solution, calcic plagioclase, Ni-rich pyrrhotite, thorite, and low-Si, Fe-rich glassy inclusions. This expands upon prior surveys; New England, eastern Australia, in particular, shows the most diverse inclusion suite yet documented from basalt-hosted corundums (20 phases). Rare earth element patterns of zircon inclusions, intergrowths, and megacrysts are similar, with the exception of variable Eu depletion. Temperature estimates derived from magnetite exsolution textures, feldspar compositions, and fluid inclusion homogenization suggest corundum crystallization occurred at 685–900 °C. The overlap in Nb–Ta oxide compositions between corundum inclusions and those found in niobium–yttrium–fluorine (NYF) granitic pegmatites supports a silicate melt origin. The MELTS thermodynamic model was applied to amphibole–pyroxenite xenolith compositions from basalts to test the viability of corundum formation from mantle-derived melts. The calculations indicate corundum crystallization occurs at 720–880 °C and 0.7–1.1 GPa, producing residual feldspathic assemblages matching observed mineral inclusions and xenoliths. A model is thus proposed in which corundum in the BGY suite forms from low-volume silicate melts generated during partial melting of amphibole-bearing lithospheric mantle under the influence of magmatic plume activity.

gems, spinel, garnet, star spinel, star garnet, spinel inclusions, garnet inclusions

Four-ray and six-ray star networks are reported in spinel and garnet from Sri Lanka. Details of their arrangement and orientations are discussed.

gems, jade, pyroxene jade, fei cui, Guatemala, Mesoamerica, Nicaragua, Mexico, Honduras, Costa Rica, archaeogemology, Cuba, Dominican Republic, geology

Jadeitite occurs along with albitite (ex-jadeitite) in blocks in tectonised serpentinite, or more often in river pebbles, along the Motagua River Valley and its tributaries (MRV) in Guatemala (e.g. McBirney et al., 1967; Harlow, 1994). The MRV is generally considered to be the source area for most, if not all, of the jade used widely in the prehispanic civilisations ranging from the Olmecs through the Mayas to the Aztecs (e.g. Hargett, 1990; Harlow, 1991). Harlow (1994) mentioned the evident spatial relationships of known jadeitite or eclogite occurrences with the major suture contact of the North American and Caribbean Plates (which runs W.S.W.-E.N.E. precisely along the MRV), and he emphasised that jadeitite and eclogite are respectively restricted to the North and South sides of the MRV. Raman Microprobe (RMP) analysis on the unprepared small (~4×2×2 cm) green river pebble n° FG96/27, collected from the South side of the MRV, showed it to be a near monomineralic jadeitite with a composition of 90 ± 10 mol% Jd, with a characteristic T-O-T vibration at 697–701 cm⁻¹ (cf. Smith & Boyer, 1985; Smith & Gendron, 1997). Both macro- and micro-mode RMP also revealed local enrichments of rutile, a mineral not listed in the comprehensive study of MRV jadeitites by Harlow (1994). A subsequently manufactured thin section revealed: an irregular granular microcrystalline texture with the pyroxene (~97 vol%, i.e. true jadeite jade) sometimes showing optically-zoned grain boundaries; abundant rutile (~2 vol%); rare titanite and zircon; and more importantly numerous micron-sized inclusions in the cores of a few of the larger equant to elongate jadeite grains. EMP analysis then proved 77 to 97 mol% Jd. All analysed microinclusions are quartz (also not listed by Harlow, 1994), thus indicating a trapped core assemblage on the higher P side of the Ab = Jd + Qz reaction for the given compositions. No other mineral species have been found, in particular no albite, nepheline nor analcime, and hence no retrogression at all. Jadeitite n° FG96/27 is thus very different from all the varieties of jadeitite from the North side of the MRV described by Harlow (1994). Therefore it seems that its occurrence on the South side is a geological reality, rather than a human-caused anomaly (e.g. prehispanic commerce). Hence it represents a new locality and a new kind of jadeitite with a higher-P different P-T-X-t trajectory, but most probably one formed by the same overall petrogenetical process and geodynamical event. It thus also presents an extra possibility for the geological provenancing of Mesoamerican artefacts in jade which, in fact, collectively reveal several distinct kinds of jade.

gems, corundum, Madagascar.12

Sapphires from northern Madagascar. In the past few years, several new and important sapphire deposits have been discovered in Madagascar. Dr. Margherita Superchi, of CISGEM, Milan, Italy, described a new occurrence at Ambondromifehy, in Antsiranana Province, in the northern part of the island. The sapphires found to date were alluvial, having been weathered from basaltic rocks that were emplaced intermittently during the period from the Upper Oligocene to the Pleistocene. The sapphires are thought to be xenocryts; that is, they did not crystallize from the basaltic magma, but simply were transported by it. They are associated with zircon, spinel, pyroxene, and olivine. Dr. Superchi and her colleagues (A. Donini, D. Muzzioli, and E. Roman) obtained detailed gemological data, chemical compositions, and optical and Raman spectra for 74 crystals (the largest being 182.59 ct) and 13 cut stones. Among the more interesting discoveries were inclusions of pyrochlore, zircon, and sodic plagioclase, which, with the chemical data (e.g., high gallium contents) confirm the “basaltic” association of these sapphires (see the earlier abstract on gem corundum).

corundum, treatments.6, gems

CMS: The Swiss Gemmological Institute (SSEF) in Basel has found its recent acquisition of a Raman microscope well worth the investment, to judge by this excellent account of its initial applications at this laboratory. Much more accurate, easier to use, and considerably less expensive than its predecessors, the Renishaw Raman microscope at SSEF easily fits on a desktop. It has enabled SSEF staff members to identify CO₂ inclusions in sapphire that are diagnostic of heat treatment, detect and identify fracture fillings in emerald, easily identify mounted gemstones, and characterize specimens with irregular surfaces. It can also distinguish component minerals in many aggregates. The method is nondestructive in all but a few well-documented conditions and works on all types of materials, except those having simple compositions and high symmetry, such as metals and alloys. Raman spectroscopy does require that the user be aware of its limitations and parameters; also, the lack of a large, published database means that each laboratory must obtain many of its own reference spectra from known samples. However, Raman spectroscopy clearly has earned a place of respect among the few high-tech laboratory techniques that have proved indispensable to gemological testing today.