gems, synthetic chrysoberyl, Russia, horizontally oriented crystallization

Mineralogical and gemmological properties of synthetic alexandrites grown by the horizontally oriented crystallization (HOC) method are described. The method is used to grow crystal plates weighing up to 450 g. The morphology of the rough crystals is dependent on the form of the molybdenum boat used as a container for crystal growth. The most significant inclusions for gem identification are curved growth striations and flat, irregularly- shaped somewhat elongated cavities. Microprobe analyses revealed distinct amounts of chromium and vanadium but extremely low iron contents. Spectra in the UV-Vis range are consistent with published work. Six differently sized cubes were cut from an HOC alexandrite crystal and used to show pleochroism in combination with two optical phenomena: the alexandrite effect and the Usambara effect. Colorimetric parameters were calculated from the spectra of the cubes and plotted in the CIELAB colour space to demonstrate the alexandrite and Usambara effects graphically for different directions of view. The absorption bands in the infrared are polarized and assigned to structural OH-groups.

corundum, Burma.12, gems

In the last five years, fine Burmese blue sapphires from the Baw Mar area of Mogok have reached the market. The faceted stones typically show a strong pleochroism from greenish to violetish blue when viewed perpendicular and parallel to the c-axis, respectively, with medium to strong saturation and medium to dark tone. Most of the samples were relatively clean under the microscope, showing multiple twinning with whitish needle-like inclusions (presumably boehmite) at the intersections. Often, these in-clusions were associated with stress tension fissures. Needles, most likely rutile, were found only occa-sionally, but small platelets and needle-like particles, probably ilmenite, appeared more frequently. Most of the stones contained surface-reaching open and healed fissures, but crystal inclusions of K-feldspar and mica (identified by Raman) were occasionally encountered. The sapphires also had a relatively high iron content, low gallium, and very low titanium. Their Ga/Mg ratio varied from 0.6 to 17. Their UV-Vis-NIR spectra displayed intense iron-related absorptions, and the FTIR absorption spectra presented mainly boehmite-and mica-related bands. Based on careful microscopic observations, combined with spectro-scopic and chemical analysis, the sapphire from Baw Mar can, in most cases, be distinguished from the blue sapphire of other localities.

corundum, inclusions.5, geology.11, Mozambique.12, gems

Rubies from Montepuez in northern Mozambique are particularly interesting as this new material rapidly dominated the ruby trade in Thailand, the world’s major ruby trading center, since the discovery of the deposit during the spring of 2009. This new material is quite different from rubies traditionally encountered in the market such as the marble type rubies, known for their strong fluorescent aspect related to their low iron content, like those from Mogok or Mong Hsu in Myanmar (Burma) or basalt related rubies, known for their low fluorescence due to higher iron content, from Thailand or Cambodia. Indeed the new deposit in Montepuez is not marble or basalt related but instead is an amphibole related deposit similar to the ruby deposits near Winza (Tanzania), M’Sawize (Mozambique) and probably the deposit near Didy (Madagascar), three other ruby deposits discovered fairly recently in Africa. Amphibole type rubies usually have higher iron content than those commonly found in marble related deposits but also usually exhibit lower iron content compared to those from basalt related deposits. It is interesting to note that these non-classic (i.e. amphibole type) metamorphic stones fill the gap, in relation to chemistry and appearance, between the highly fluorescent marble type rubies and the weakly fluorescent basalt related rubies familiar to the trade. For gemologists willing to find out where the gems they are studying come from, the fact that the rubies from Montepuez are coming from an amphibole related deposit is fascinating as the combination of the inclusion scene, the chemical fingerprint and the spectra associated with these stones will usually enable them to differentiate these gems quite easily from their cousins mined from marble or basalt related deposits. The data presented here, using GIA reference samples, should be very helpful in this regard.

corundum, inclusions.5, treatments.7, gems

gems, jade, pyroxene jade, jadeite, fei cui, Dominican Republic

New occurrences of jadeitite and jadeite-rich rocks have been discovered in the Río San Juan Complex (RSJC) of the northern Dominican Republic within serpentinite mélanges associated with a former intra-oceanic subduction zone. Allochthonous blocks occurring in lag deposits developed upon mélange outcrops, as well as boulders within riverbeds, are common. A particularly unusual feature of the RSJC is the presence of concordant layers and discordant veins, ranging from centimetres to decimetres in thickness, within blocks of jadeite-lawsonite- or omphacite-garnet-bearing blueschist in the mélange. Two suites of jadeite-rich rocks may be distinguished. The first consists of quartz-free jadeitite sensu stricto (>90 vol.% jadeite), so far known only from blocks and boulders. The second suite comprises quartz-bearing jadeitite sensu stricto grading into jadeitite quartzite (JQ), jadeite-lawsonite quartzite (JLQ) and jadeite-free lawsonite quartzite (LQ). This second suite occurs not only as blocks and boulders but also as layers and veins within blueschist blocks. A single occurrence of a cross-cutting omphacitite vein within blueschist has likewise been documented. Additional minerals identified within both suites include omphacite, phengite, glaucophane, epidote, albite, calcite, titanite and zircon. Apatite and pumpellyite have thus far been observed only within the quartz-free jadeitite sensu stricto, whereas almandine-rich garnet has been found exclusively within JLQ. The jadeite-bearing rocks occur in various shades of green, range from fine- to coarse-grained and are generally equigranular. Mineral distribution is commonly homogeneous, although JLQ may display a patchy distribution that imparts a distinctly mottled appearance. Cathodoluminescence imaging reveals oscillatory zoning in jadeite, zircon, apatite and calcite, indicating crystallization from aqueous fluids under open-system conditions. Zircons separated from quartz-free jadeitite sensu stricto contain primary inclusions of high-pressure matrix minerals such as jadeite and omphacite, demonstrating coeval zircon growth. Zircon cores yield ages of 114.9 ± 2.9 Ma, indicating crystallization near the initiation of subduction within the Río San Juan Complex, at a time when relatively “warm” geothermal gradients of approximately 15°C/km prevailed. These ages contrast sharply with those of the blueschists hosting the younger quartz-bearing suite, which range from 80 to 62 Ma and correspond to the final stages of subduction-zone activity at around 55 Ma under cooler geothermal gradients of approximately 8–9°C/km. For the younger quartz-bearing suite, phengite compositions combined with available P–T–t paths for the host blueschists suggest crystallization temperatures of approximately 350–500°C at minimum pressures of 15–16 kbar. Conditions for the older quartz-free suite are more difficult to constrain, although phengite compositions together with the geothermal conditions prevailing in the early warm subduction zone imply minimum conditions of at least 500°C and 11 kbar. Temperatures and pressures as high as 600°C and 15 kbar, however, comparable to those documented for jadeitites of similar age exposed in eastern Cuba within the same subduction system, remain possible. The jadeitites and jadeite-rich rocks of the RSJC are therefore interpreted as having crystallized over a span of approximately 60 million years within a single thermally self-organizing and progressively cooling subduction zone, evolving from initial temperatures exceeding 500°C to later conditions near 350°C. The P–T conditions inferred for the younger quartz-bearing suite correspond closely with those of jadeitite formation south of the Motagua Fault Zone in Guatemala, the only other known locality worldwide where jadeitite containing both lawsonite and quartz appears to be common. Further evidence is nevertheless required to confirm whether the older quartz-free suite represents another example of the rare high-temperature jadeitite known from Cuba.

gems, beryl, emerald, Ethiopia

At the 2011 Tucson gem shows, gem dealer Farooq Hashmi (Intimate Gems, Glen Cove, New York) loaned the GIA a 1.36 ct emerald reportedly sourced from near Dubuluk, a town in southern Ethiopia approximately 80 km north of the Kenyan border. Hashmi noted that the area had been producing emeralds for a few years, and during a buying trip to Ethiopia in 2011, he observed several hundred grams of rough material, including some large crystals, though gem-quality portions were typically small. Examination of the trilliant-cut emerald revealed green color with weak pleochroism (yellowish green and bluish green), refractive indices of no = 1.585 and ne = 1.578, a birefringence of 0.007, specific gravity of 2.72, and weak red fluorescence under long-wave UV. Strong absorption lines near 700 nm were visible in a desk-model spectroscope, consistent with known emerald properties. Microscopic inspection showed roiled and jagged growth structures, blocky two-phase fluid inclusions, pale brown plates, short needles, and a pale brown elongated inclusion. Electron microprobe analysis conducted at the University of New Orleans indicated 0.05 wt.% Cr₂O₃ and 0.10 wt.% FeO, with vanadium below detection limits. Polarized UV-Vis-NIR spectroscopy revealed absorption features due to both Fe²⁺ and Cr³⁺, while the infrared spectrum showed bands at 2358 cm⁻¹ (linked to CO₂ in structural channels) and 2290 cm⁻¹ (of unknown origin). These findings suggest that Ethiopia may soon add emerald to its expanding portfolio of commercially significant gemstones.

corundum, origin determination, chemistry.2, inclusions.5, gems

corundum, color.4, inclusions.5, treatments.6, gems

A time-dependent phase stability diagram has been developed describing the structural evolution of TiO₂ precipitates in Ti-doped sapphire (α-Al₂O₃). A high-pressure TiO₂ phase with the α-PbO₂ structure (α-TiO₂) forms as a coherent precipitate in Ti-doped sapphire annealed at 1200°–1300 °C; extended annealing times at these temperatures result in twinned particles, the twinning minimizing the precipitate–matrix misfit strain energy. Twinned α-TiO₂ eventually transforms to the rutile structure, coincident with the loss of coherency with the sapphire matrix. Rutile nucleates directly in sapphire annealed at temperatures at 1350 °C and above. Rutile is present mainly as semicoherent and twinned precipitates, the twinning occurring relatively early in the precipitation sequence. RWH: Great paper on the formation of rutile silk in corundum.

corundum, inclusions.5, Thailand.12, Laos.12, gems

Mineral inclusions and coexisting minerals of gem corundum from the Thailand area analysed by SEM-EDS in this study can be divided into sapphire and ruby suites. The sapphire suite includes alkali feldspar (sanidine), nepheline, zircon, hercynite-spinel series and magnetite-hercynite series. The ruby suite includes garnet (pyrope), high-Al diopside ( fassaite') and sapphirine. The mineral chemistry and other data suggest that Thai rubies could have crystallized in high-pressure metamorphic rock of ultramafic/mafic composition. In contrast, Thai area sapphires could have crystallized in high-grade metamorphic (gneissic) rock or from magmas of highly alkaline composition, located at shallower levels in the lithosphere than those hosting the Thai rubies.

gems, opal, Ethiopia

A new opal deposit discovered in 2008 near the village of Wegel Tena in Ethiopia’s Wollo Province has yielded mostly white opal, along with some brown, fire, and colorless “crystal” opal, all hosted in volcanic rocks. While some specimens resemble sedimentary opals from Australia and Brazil due to their vivid play-of-color, their properties align with opal-CT and typical volcanic opals. Inclusions of pyrite, barium-manganese oxides, and native carbon are present, and some samples feature unusual “digit patterns,” where play-of-color and common opal interpenetrate like fingers. These opaque-to-translucent opals exhibit pronounced hydrophane behavior, becoming transparent when soaked in water. Notably, the white opals show elevated barium levels, previously unreported in opal-CT. Though the fire and crystal varieties are fragile, the white opals demonstrate exceptional durability. With an unusually high proportion of gem-quality material and over 1,500 kg already extracted using basic mining methods, this deposit—possibly extending several kilometers—has the potential to become a major source of fine opal.