alexandrite, beryl, emerald, ruby, sapphire, topaz, biotite, phlogopite, Australia, gems, chrysoberyl, beryl

An unusual association of alexandrite, emerald, ruby, sapphire and topaz is described from Poona, near Cue in Western Australia, following field investigations in 1994. The discovery established the first documented occurrence of alexandrite-bearing bedrock in Australia. The minerals occur as preserved or fragmented porphyroblasts within biotite–phlogopite rocks belonging to a metasomatic blackwall sequence. Three types of emerald mineralization are distinguished: emerald associated with margarite and topaz; emerald accompanied by ruby, sapphire, topaz and alexandrite; and emerald associated with quartz and feldspar, with or without garnet and muscovite. The second assemblage, found in specimens from the dumps of the “Ruby shaft” near the Aga Khan Deep Mine, contains millimetre- to centimetre-scale bands interpreted as metasomatically altered greisens. Pink to violet fluorite, white micas and quartz accompany the gem minerals. Corundum commonly forms tabular crystals and radial or subparallel aggregates, with blue or colourless sapphire locally preserved within younger pink ruby. Many crystals display a deep blue core, a relatively colourless intermediate zone and a red outer zone, suggesting increasing chromium availability during growth. Alexandrite occurs as small prismatic crystals and characteristic star-like twinned aggregates, generally green to blue-green in daylight and blue to violet-red under incandescent illumination. Ruby and alexandrite fluoresce red under ultraviolet radiation, more strongly under long-wave than short-wave excitation, whereas colourless and blue corundum show no fluorescence. Corresponding colour zoning in corundum, chrysoberyl and beryl supports a multistage growth history involving changes in the supply of colouring elements. The proposed genetic model begins with fluorine-, beryllium- and rare-metal-bearing fluids from granites entering Archaean ultramafic rocks and producing greisens and mineralized veins. Subsequent regional metamorphism at upper greenschist to lower amphibolite facies promoted metasomatic reactions between these veins and their host rocks, forming corundum, alexandrite and emerald. During retrogression, topaz, corundum and chrysoberyl were partly or completely replaced by margarite, muscovite and locally emerald. Although the assemblage offers exploration potential, most crystals examined were small, fractured, inclusion-rich or altered, and little material suitable for faceting had been recovered. The economic prospects for ruby and alexandrite therefore remained uncertain.

jade, jadeite, fei cui, zircon, inclusions, gems

As the practice of bleaching and impregnating jadeite has become more common over the last few years, the amount of jadeite we test in the laboratory has increased signifi-cantly. During the microscopic exam-ination of one variegated green jadeite cabochon for possible evidence of treatment, we noted that the stone contained a number of small crystal inclusions, some of which reached the surface (figure 4). These surface-reaching crystals appeared to be somewhat harder than the surround-ing jadeite, and in reflected light they had a higher surface luster. With the recent acquisition of GIA’s Renishaw Raman Imaging Microscope System, we were able to examine two of these crystals nondestructively. By focusing the argon laser through the 50 x objective of the Leica targeting microscope and run-ning several continuous scans, we obtained clear spectra of the two “inclusions.” A computer search of our data base revealed that these spec-tra matched the spectrum for zircon. Visible characteristics of the inclu-sions (e.g., crystal form, color, trans-parency, and relative hardness) were consistent with this identification. This is the first time that we have encountered zircon crystals as inclu-sions in jadeite. Coincidentally, about a month prior to our discovery, a zir-con inclusion in a jadeite cabochon was also identified by means of the same technique at the Center for Gemstone Testing in Bangkok (Ken-neth Scarratt, pers. comm.).

corundum, inclusions.5, Sri Lanka.12, gems

Carbonin, Sbrignadello, and Ajò demonstrate the value of combining independent analytical techniques to identify solid inclusions in corundum through a study of a commercially obtained blue sapphire containing thorite. The stone was provisionally assigned to a Sri Lanka type on the basis of an experienced supplier’s assessment, but reliable information establishing its geographical provenance was unavailable. Transmitted-light microscopy revealed at least two visually distinct inclusion types. A red, well-formed octahedral crystal associated with fingerprint inclusions was considered to be chromium-bearing spinel, a feature previously described in Sri Lankan sapphire. Photoluminescence measurements were undertaken with helium–neon laser excitation and subsequently with a more intense argon laser because the initial chromium-related emission from the host sapphire was weak and noisy. The principal identification problem concerned a second inclusion, approximately 45 µm long, whose habit, surrounding tension haloes, and fissures initially suggested metamict zircon. Because this grain lay close to the sapphire’s surface, the host was polished until the inclusion was exposed for electron microprobe examination. Energy-dispersive analysis unexpectedly detected no characteristic zirconium radiation. Instead, silicon and several diagnostic thorium emissions dominated the spectrum, supporting identification as thorite, ThSiO₄, whose crystal habit closely resembles that of zircon. The grain’s extremely small size prompted confirmation by an independent method. Low-background gamma spectrometry using an intrinsic germanium detector established the presence of natural thorium-232 through characteristic emissions from its radioactive daughter products, lead-212 and thallium-208, at 238.6 and 583.1 keV, respectively. The agreement between elemental microanalysis and nuclear spectrometry strengthened the thorite identification beyond what could be inferred from optical appearance alone. The authors emphasize that their complementary methods probe different physical processes, involving electronic and nuclear energy levels, and therefore provide independent support when a microscopic identification is uncertain. The study cautions against assigning zircon solely from crystal morphology and associated stress features, since another radioactive silicate can present a similar appearance. It also suggests that thorite should be considered among the inclusion species potentially relevant to sapphire provenance investigations, while the uncertain source of the specimen prevents its use as firm evidence of a Sri Lankan occurrence.

corundum, synthetic corundum.7, gems

Hydrothermal synthetic rubies marketed under the name “JL synthetic rubies” are available from Guangxi Jewelry Co. Ltd. in Guilin, China (Guangxi Province). The material ranges from red to orange-red in color, with some displaying purple hues. The synthetic rubies fluoresce strong red to long-wave ultraviolet radiation; the fluorescence is less intense to short-wave UV. Colorless “net-like” inclusions are reported. The synthetic rubies have R.I. values of 1.759–1.769, and an S.G. of 4.01. They are incorporated into gold rings, pendants, earrings, and bracelets. MM

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.

alexandrite, beryl, emerald, phenakite, Franqueira, Spain, gems

Marcos-Pascual and Moreiras examine the structural, chemical, physical and microscopic characteristics of emerald, alexandrite and phenakite from Franqueira in northwestern Spain, documenting an unusual beryllium-mineral association and assessing its gemological potential. Mineralization occurs in phlogopite-rich metasomatic rock developed where fluid-rich granitic pegmatites interacted with dunite. Beryllium supplied by the pegmatites combined with chromium derived from the ultramafic rock to produce emerald and alexandrite. The study employs single-crystal X-ray diffraction, electron microprobe analysis, refractometry, hydrostatic weighing, infrared spectroscopy and microscopy, supplemented by determinations of lithium, beryllium and water in emerald. Emerald is the most abundant beryllium mineral, forming light to medium green prismatic crystals up to 10 cm long. Most material is translucent and unevenly coloured, although distinct growth-related colour zoning was not observed. Chemical and structural data indicate predominantly octahedral substitution in the beryl structure, with channel alkalis contributing to charge compensation. Infrared spectra show that alkali-associated Type II water predominates over Type I water. Common inclusions comprise brown iron-rich phlogopite, negative crystals containing fluid phases, and healed fractures; less frequent mineral inclusions include quartz, pyrite, phenakite, alexandrite, ilmenite and anthophyllite. Previously reported fluid studies distinguish methane-bearing, carbon-dioxide-bearing and secondary saline aqueous inclusions. Alexandrite forms twinned crystals and skeletal inclusions in emerald, displaying a pronounced change from green or bluish-green in daylight to purplish-red under incandescent illumination, together with strong pleochroism. Abundant phlogopite inclusions and fractures limit the availability of good-quality cut stones. Phenakite is less abundant but can be transparent, colourless and suitable for faceting; its inclusions also include phlogopite and fluid-bearing healed fractures. Comparison with the genetically similar Tokovaja deposits in the Ural Mountains reveals broadly comparable alexandrite and phenakite properties, but differences in emerald colour, chemistry, infrared band intensities and inclusion morphology. Franqueira emeralds are generally paler because of lower chromium contents and lack several channel-like inclusion structures described in Uralian material. The authors relate these differences partly to the much smaller exposed extent and narrower geological diversity of Franqueira, while allowing that mineralization may continue underground. The study establishes a reference for identifying and understanding material from a locality vulnerable to depletion by collecting.

corundum, inclusions.5, gems

The first phase to precipitate in Ti-doped sapphire (Ti:α-Al₂O₃) single crystals annealed at 1300 °C in air is a high-pressure form of TiO₂ with the α-PbO₂ structure (α-TiO₂). The coherent precipitates form as oblate spheroidal plates with habit planes parallel to (0001). Because of a 5% lattice mismatch with sapphire, the small coherent α-TiO₂ precipitates are under an effective compressive stress, which stabilizes the high-pressure polymorph. During coarsening, the particles lose coherency by emitting a dislocation loop with a f(1̄101) Burgers vector, which reduces the stress on the particles and allows them to transform to the rutile structure. The orientation relationship between the sapphire matrix, α-TiO₂ and rutile has been determined and a lattice correspondence for the phase transformations established. Although the transformation from α-TiO₂ to rutile is purely structural, its rate is controlled by atomic diffusion; the increased atomic volume of rutile compared with α-TiO₂ is accommodated by expelling extra mass of Al₂O₃ into nearby interstitial dislocation loops and is associated with a change in the precipitate morphology.

corundum, chemistry.2, sources.12, Myanmar (Burma).12, gems, trace element analysis, ruby

The authors analyzed 130 untreated rough rubies from nine metamorphic localities in Myanmar, including Mogok and Mong Hsu, using micro proton-induced X-ray emission (PIXE). The concentrations of chromium, titanium, vanadium, iron, copper, and gallium—the most abundant trace elements—were determined for a 200 × 200 μm area on each stone. In a unique approach, the authors used the trace-element mapping capability of the instrument to eliminate inhomogeneities, such as inclusions, from the analyses for the areas examined. The measurements revealed that, with the exception of gallium, there were large variations in the trace-element chemistry of rubies from the different localities, as well as for rubies from the same locality. Nevertheless, some consistent differences were also found. Using statistical analysis, including tree clustering and factor analysis, the authors determined that the origin of rubies from six of the nine localities could, indeed, be determined using this technique.

corundum, sources.12, Australia.12, , gems

Ruby is rare in Australia, but in the Barrington area of New South Wales, ruby and pink sapphire form up to 50% by weight of the gem corundum concentrate. These stones have Cr₂O₃ contents of 0.60–1.42 wt.% and FeO contents of 0.44–0.75 wt.%. They typically range from 2 to 6 mm in size and contain inclusions of spinel and liquid-filled fractures. Polysynthetic twinning is common and is often associated with inclusions of boehmite needles. On the basis of trace-element content, the Barrington corundum is divided into two distinct suites: one related to ruby and the other typical of eastern Australian sapphires. The ruby-containing corundum suite originated in a metamorphic environment, whereas the non-ruby suite has a magmatic origin. Both suites were brought to the surface by basaltic volcanism.

gems, Sri Lanka, sapphire, fluid inclusions

Examines fluid inclusions in sapphires from Myanmar, Kashmir, Sri Lanka, Madagascar, and Rwanda using microthermometry and Raman spectroscopy. The fluids consist of carbon dioxide, while diaspore daughter crystals indicate that water was formerly present. Four inclusion types are distinguished by their morphology, occurrence, and fluid density. Primary inclusions form well-developed, approximately equidimensional negative crystals containing high-density fluid. Secondary inclusions include short prismatic cavities within well-preserved healed fractures, together with two further types associated with incompletely healed fractures. Differences in the densities of primary fluid inclusions allow sapphires from Sri Lanka and Kashmir to be distinguished from those of the other localities examined, demonstrating the potential of fluid-inclusion analysis as an aid to sapphire characterization.