gems, inclusions, beryl, emerald, Ethiopia

In 2016, a new deposit of emeralds was discovered in the rural village of Kenticha and Dermi in the Seba Boru distinct near the town of Shakiso, which is located in the Oromia province in the Southern Ethiopia. GIA had the opportunity to study many samples from this new deposit.

gems, corundum, inclusions.5, Burma.12

The Mong Hsu ruby deposit in Myanmar is a marble-type deposit. It is well-known for attractive crystals with dark violet/blue to black cores and ruby rims, as well as for trapiche rubies. These exceptional rubies have a texture composed of six growth sectors separated by six dendrites running from a central core to the six corners. Fluid inclusions (FI) have been studied in both types of ruby, i.e., trapiche and non-trapiche crystals. Microthermometry combined with Raman spectrometry investigations of primary and secondary FI proved the existence of CO₂–H₂S–COS–S₈–AlO(OH)-bearing fluids with diaspore and native sulfur daughter minerals. The carbonic fluid belongs to the CO₂–H₂S system with CO₂ as a dominant component. Minor COS (1–2 mol.%) and diaspore indicate that H₂O (∼1 mol.%) was present in the paleofluid. The amount of H₂S in FI from trapiche ruby is between 6.9 and 7.4 mol.% for the dendrites and from 7.2 to 15.7 mol.% for the growth sectors. The H₂S content is approximately 10 mol.% in FI from non-trapiche rubies. The presence of dawsonite and previous crushing data carried out on well-cleaned crystals of non-trapiche ruby indicate that an ionic salt melt was present at the time of ruby formation. The non-detection of this melt in trapiche ruby is attributed to the small size of the remnants of molten salts, making it difficult to observe them under the microscope. Following this hypothesis, the FI assemblages in rubies would correspond to the trapping of two immiscible fluids, i.e., a carbonic phase in the CO₂–H₂S–COS–S₈–AlO(OH)-system and molten salts. Such fluid phases are considered to be the product of metamorphism of evaporites during devolatilization of carbonates and thermal-sulfate reduction. The formation of trapiche ruby is discussed in terms of possible variations of the composition of the fluid, temperature, and pressure conditions. The variation in density of the carbonic fluids from the dendrites (0.70 < ρ < 0.78) to the growth sectors (0.56 < ρ < 0.68) records variation of fluid pressure in the metamorphic system. This variation led to episodically local fluid overpressure and hydraulic fracturing in the marble. During such episodes, changes in driving-force conditions allowed for the formation of the trapiche texture in ruby: the development of dendrites and growth sectors occurred under high and low driving-force conditions, respectively. Non-trapiche ruby in veinlets formed concurrently under thermodynamic conditions similar to those registered for the growth sectors in trapiche ruby.

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

Recently, a 5.06 ct yellow sapphire was submitted to the American Gemological Laboratories (AGL) for testing (Figure 18). Standard gemmological analyses distinguished that this was an unheated yellow sapphire originating from Sri Lanka. What caught the eye of this gemmologist were two well-formed octahedral crystals situated under the table of the gem (Figure 19). Raman analyses identified them as spinel. Corundum and spinel often form in similar geological environments and geographical locations. Given this, it is not surprising that spinel (as well as gahno-spinel) has been identified previously in gem-quality corundum from a variety of sources, such as Sri Lanka and Myanmar/Burma (see, e.g., Gübelin and Koivula, 2008). However, it is not as common to see such euhedral colourless crystals of spinel as are present in this yellow sapphire.

gems, corundum, geology.11, Vietnam.12, spinel

Corundum deposits in the Tan Huong–Truc Lau area of Vietnam are hosted mainly in gneiss and partly in feldspathic pegmatoid and marble. Encountered here in both primary and secondary placer deposits is a considerable quantity of stones with a well-developed spinel corona following the morphology of the corundum core. The genesis of the observed spinel corona can be explained by their reaction textures, chemical composition, and mineral inclusions as well as by applying thermodynamic phase equilibria calculations. The following reactions were found to be responsible for the formation of the spinel corona around corundum in gneiss: biotite + corundum → spinel + K-feldspar + fluid, or garnet+corundum → spinel + sillimanite For corundum-spinel associations in metacarbonate rocks, the relevant reaction is: dolomite + corundum → spinel + calcite + carbon dioxide. Mineralogical and chemical studies have shown that the gem quality of ruby and sapphire grains from Tan Huong–Truc Lau deteriorated substantially after their formation due to this corona texture.

corundum, Mozambique.12, ruby, inclusions.5, treatments.6, gems

Gemstones have been treated for as long as they have been used by humans. The supply of untreated high-quality gems is so incredibly small that people have always tried to find ways to optimize the other material found in the mines. Treatments come in many different forms and degrees of quality improvement. Dyeing to create color, oiling to hide fractures, heat treatment to increase clarity and optimize color, irradiation to modify color, and impregnation for better stability are just a few examples (Nassau, 1981). In the case of corundum, heat treatment is the most common form. By applying the correct heating process, color can be introduced, strengthened, reduced, or removed. These variations are determined by an interplay between the gemstones, temperature profile during treatment, duration of heating, heating atmosphere (reducing or oxidizing), and the presence of added chemicals (Hughes, 2016; Emmett et al., 2003, 1993). Recent developments have caused drastic changes in heat treatment applications. Since 2009, Mozambique has been an important source of rubies in the world trade. It was quickly discovered that these stones reacted well to a form of heat treatment that used lower temperatures. This modern treatment at low temperatures is distinguished from the classic high-temperature methods by comparing their effect on the minute particles in the stones. These second-phase microcrystals are most commonly rutile (Ti-oxide) that takes the form of fine clouds or silk. During low-temperature treatment, these inclusions are not dissolved in the corundum lattice. This is clearly the case for high-temperature treatment, where silk is obviously altered or even completely diffused. Since heat treatment depends on more than the temperature alone, it is very hard to state a clear temperature boundary between low-temperature and high-temperature treatment. The most important factors to consider in this case are temperature and time. The speed of the process increases considerably with higher temperatures, but longer heating periods at lower temperatures might yield similar results. A rough guideline for the temperature boundary is somewhere between 1200 and 1350°C. Preliminary studies on low-temperature treatment have been published (Pardieu, Saeseaw et al., 2015; Sripoonjan, Wanthanachaisaeng et al., 2016). Both of these publications mention subtle alterations, notably in stained fractures and silk platelets. Spectroscopy showed promising results, but the results were inconclusive due to the small sample sizes. Because heat treatment processes are constantly evolving, updated data needs to be collected regularly. The objective of this project is to observe changes in Mozambican rubies during known treatment conditions (temperature and duration). The focus is on inclusions and FTIR spectra after heat treatment.

gems, corundum, inclusions.5, Ethiopia.12

gems, spinel, spinel inclusions, chemistry, SEM, Vietnam, Sri Lanka, corundum, zoisite, allanite, paragonite, k-feldspar, plagioclase, florencite, biotite, pyrite, diopside, dolomite, calcite, titanite, rutile, zircon, graphite, pyrrhotite, pentlandite, violarite, clinochlore, phlogopite, magnetite, chondrodite

A study on the chemistry and gemmological features of spinels from Ratnapura in Sri Lanka and Luc Yen in Vietnam using scanning electron microscopy. The research identifies chromophore oxides responsible for coloration, noting that V³⁺ determines yellow and brown hues, Cr³⁺ drives red and pink tones, and Fe²⁺ with low Co²⁺ produces purple or violet shades. Mineral inclusions were cataloged, with Vietnamese spinels containing dolomite, calcite, titanite, rutile, zircon, graphite, apatite, pyrrhotite, pentlandite, violarite, clinochlore, phlogopite, magnesite and chondrodite, while Sri Lankan samples featured allanite, paragonite, zoisite, K-feldspar, plagioclase, florencite, biotite, pyrite and diopside. The authors conclude that vanadium oxide content above 0.4 wt.% is critical for orange and brown coloration in spinels, complementing existing models that primarily rely on chromium, iron and cobalt.

gems, garnet, India, history of gemology, archaeogemology, archaeology, Indian lapidary

The archaeological site of Arikamedu, located in Tamil Nadu State on the east coast of India, was for many centuries the center of a significant bead-producing industry. Beads were made from both glass and stone, including garnet, though the source of the garnet rough material had not been confirmed. To investigate this, garnet beads found at Arikamedu were compared with rough material from the Garibpet deposit, located approximately 640 km away in Telangana State, east of Hyderabad. Samples from both locations showed substantial similarities in average composition, trace-element contents, chemical zoning of major and minor elements, inclusion assemblages, and zoning of inclusions between the rims and cores of the crystals. Chemically, the garnets were almandine-rich (averaging 81.0% almandine, 11.5% pyrope, 3.3% spessartine, and 1.5% grossular), with clear zoning for manganese (Mn) and magnesium (Mg), as well as trace elements such as yttrium (Y), phosphorus (P), and zinc (Zn). The most distinctive aspect of the inclusion pattern was the presence of sillimanite fibers concentrated in a zone between the inclusion-rich core and the inclusion-poor rim. Combining microscopic observations, inclusion identification, and chemical analyses, it was determined that the rough material used to produce garnet beads at Arikamedu historically originated from the Garibpet deposit. Additionally, the results suggest that current classification systems for historical garnets need further refinement.

gems, jade, amphibole jade, nephrite, China

The Hetian nephrite belt in Xinjiang, China, is the world’s longest nephrite belt, stretching approximately 1,300 km, and now includes a recently identified type of brown nephrite found in Pishan County. This nephrite occurs in a skarn zone between Changchengian dolomitic marble and Late Hercynian granodiorite, formed by metasomatic processes at the contact. Microscopically, the brown nephrite consists mainly of fibrous tremolite with minor associated minerals, and its brown coloration is structurally disseminated, appearing in fissures as veinlets or leaf-like forms. EMPA and XRD analyses show it is composed of tremolite along with illite, Mg-olivine, diopside, chromite, and other accessory phases. Tremolite developed through metasomatism with dolomitic marble and by replacing diopside. Whole-rock geochemistry reveals strong negative Eu anomalies (δEu = 0.09–0.28), depleted LREEs, flat HREEs, and low overall REE concentrations, patterns closely resembling those of the host dolomitic marble, supporting a genetic link. Stable isotope data from ore-forming fluids at 330 °C (δD = –81.0‰ to –84.0‰; δ¹⁸O = 3.16‰ to 5.48‰) align with those of marble-type nephrite and differ markedly from serpentine-type nephrite, suggesting that the fluid was a mixture of magmatic and meteoric water with CO₂ from decarbonated marble. Ar–Ar dating of phlogopite indicates a formation age of 285.23 ± 1.57 Ma, while SHRIMP U–Pb dating of zircon inclusions gives an older age of 456 ± 7 Ma, suggesting that the nephrite formed after the incorporation of pre-existing zircon, supporting a Late Hercynian metasomatic origin.

gems, jade, jadeite, fei cui, jadeitite, Japan, fei cui, geology

Jadeite–quartz rocks from the Yorii area in the Kanto Mountains of central Japan occur as tectonic blocks within a serpentinite mélange. Primary two-phase (liquid + vapor) aqueous fluid inclusions have been identified in jadeite, quartz, albite, and zircon. The fluids trapped in jadeite and quartz consist of H₂O, with or without CH₄, and display a broad range of salinities—4.7 ± 1.1 wt% NaCl equivalent in jadeite, and 3.4 ± 0.7 and 13.6 ± 2.0 wt% NaCl equivalent in quartz. This study analyzes the chemical composition of fluid inclusions in quartz from jadeite–quartz rocks to elucidate the nature of fluids involved in rock formation within a subduction zone. Trace element analysis using LA–ICP–MS reveals that these fluid inclusions are enriched in large-ion lithophile elements (LILE), lithium (Li), boron (B), high field strength elements (HFSE), and transition metals. The fluids show LILE-enriched and relatively HFSE-depleted patterns, resembling the signature of HFSE-depleted fluids released from antigorite during subduction, although the absolute concentrations differ. These signatures may represent common chemical characteristics of subduction-zone fluids. Two possible scenarios are proposed for the chemical evolution of these fluids following the formation of jadeite–quartz rocks: (1) HFSE may be sequestered into the jadeite–quartz assemblage, resulting in evolved fluids that are relatively depleted in HFSE and enriched in LILE before migrating into the mantle wedge; (2) the fluids may remain in chemical equilibrium with the jadeite–quartz rocks, retaining similar compositions to the inclusions observed. In either case, the metasomatic formation of jadeite–quartz rocks produces residual fluids enriched in LILE and depleted in HFSE. This geochemical behavior mirrors that of arc magmas, which are typically LILE-enriched and HFSE-depleted. The observed elemental signatures in jadeite–quartz fluid inclusions thus provide a mechanism to explain the characteristic geochemistry of magmas in subduction zones.