gems, spinel, inclusions

Surface-reaching fissures commonly host precipitates of epigenetic minerals, as was the case with a 4.22 ct purple spinel recently examined by the author. “Islands” of unidentified birefringent inclusions occupied a near-planar fissure. These inclusions were inert to Raman spectroscopy. Delicate depositions radiating from these islands interacted to create a complex moiré pattern (see above). Moiré patterns, named after their resemblance to a type of fabric, form when parallel or concentric lines overlap. It is unclear whether the islands formed first and were partially dissolved in a secondary event, or if they formed simultaneously with the spinel. Although moiré patterns have been observed in partially healed fluid fingerprints and surface-reaching fissures, it is rare to see them expressed in such a spectacular form.

gems, jade, chalcedony, green chalcedony, causes of color

This paper presents research carried out on 10 samples of blue and green chalcedony to analyse and further discuss the cause of their colour. The gemmological characteristics of the samples are analysed using standard gemmological testing, energy-dispersive spectroscopy (EDS) testing, infrared spectroscopy testing, Raman spectroscopy and ultraviolet–visible spectroscopy testing. The results are as follows: The colour of the samples is mainly blue and green, with obvious shades accompanied by auxiliary tones such as yellow and grey; the spot measurement refractive index is 1.53–1.54; density is 2.59–2.63 g/cm³; fluorescence is inert; under crossed polarisers, the samples all show bright and extinction; under magnification, the structure is observed to be fine, with floccule-like inclusions, dark inclusions and watermark-like inclusions (Ed. Similar to left 3rd set of photos in p. 83 of 2022 GAHK Journal). The infrared spectrum shows the vibration peaks of typical quartzite; in addition to the quartz vibration peak, the Raman spectrum also shows the characteristic peak of 500 ± cm⁻¹ clinosilicate, and the dark blue-green sample also has a weak characteristic peak of chrysocolla. X-ray energy spectrometer (EDS) and ultraviolet–visible spectrum analysis further confirmed that the cause of colour in the light green sample was Ni; in the light blue, light grey-green, light yellow-green and dark blue-green samples it was Cu, with the dark bluish-green colour also caused by silicon impurities of chrysocolla; the yellowish-green and dark green samples were all artificially dyed by Cr salt.

gems, corundum, Thailand.12

The geographic origin determination of ruby is increasingly important in the gem trade and geological research. Unlike metamorphic-related ruby, the rarer basalt-related ruby has gained significant attention, especially from Thailand, a major producer of such gems. Thai rubies are classified as magmatic-related origin rubies, which can be found as xenocrysts (xenoliths) hosted by alkali basalts. This paper focuses on the gemological characteristics, inclusion morphology, identification, and geochemistry of basalt-hosted ruby from the Chanthaburi–Trat area in Thailand. Various instruments, including conventional gemological tools, Raman spectrometer, EPMA, and LA-ICP-MS, were used for analysis. This study aimed to identify the distinctive characteristics of rubies from Thailand and to find feasible methods for their geographic origin determination in comparison with rubies from Cambodia, Myanmar, and Mozambique. Thailand samples exhibit diverse inclusion scenes and contain a variety of crystal or mineral inclusions. Raman spectroscopy results indicate the presence of anorthite, titanium oxide, and gypsum inclusions. The main chemical composition of the ruby consists of Al₂O₃, with trace elements including Fe, Cr, Si, Mg, Ti, Ga, V, Ca, and Ni. The color of Thailand ruby is correlated with the content of Cr and Fe. Chemical diagrams illustrating the contents of Fe, Mg, Cr, V, Ti, and Ga offer reasonable discrimination tools for differentiating rubies from various deposit types. The chemical compositions and inclusion characteristics of rubies from Thailand serve as reliable indicators for their origin identification. This study is an advantageous supplement to the research on Thailand rubies.

gems, spinel, inclusions, Tanzania

Brief gemological description of low-grade ruby from Liberia.

gems, corundum, inclusions.5, treatments.6

Negative crystals are a common inclusion in corundum and often rupture upon exposure to moderate heat treatment. The author recently analyzed a sapphire containing a series of negative crystals arranged in an arrow-like pattern (see above). Careful examination revealed a partially healed fissure intersecting one of the negative crystals, indicating the stone had been subjected to heat treatment. Fortunately, this spectacular pattern of negative crystals endured the heat treatment process, decorating the internal micro-world of this sapphire.

gems, jade, jade imitations, glass

Libyan Desert glass is a light green or yellow natural glass which is composed of almost 98% silica. It is a type of tektite formed by meteorite impact in the desert and the fast quenching of the silica-rich melt produced. It was first reported by Clayton and Spencer in 1934 (F. Fröhlich et al., “Libyan Desert Glass: New field and Fourier transform infrared data,” Meteoritics and Planetary Science, Vol. 48, No. 12, 2013, pp. 2517–2530) and is found in Egypt’s Western Desert. Common inclusions are bubbles, cristobalite, dark brown streaks, and black iron oxides (J.A. Barrat et al., “Geochemistry and origin of Libyan Desert glasses,” Geochimica et Cosmochimica Acta, Vol. 61, No. 9, 1997, pp. 1953–1959). Recently, GIA’s Hong Kong laboratory examined a transparent light yellow bangle bracelet measuring 66.12 × 13.10 mm and weighing 144.67 ct (figure 1). It had a spot refractive index of 1.44 and revealed weak yellow fluorescence in short-wave UV. The chemical composition, determined by qualitative analysis using energy-dispersive X-ray fluorescence, was nearly pure silica with minor amounts of iron, strontium, potassium, titanium, and zirconium. Based on the differences in molecular vibrations detected with Fourier-transform infrared (FTIR) analysis, it was determined that the molecular structure of Libyan Desert glass is significantly different from that of other pure silica glasses. It has a higher ratio of discontinuities and defects in the tetrahedral (SiO4) network (F. Fröhlich et al., 2013). Microscopic observation revealed common inclusions in glass such as numerous gas bubbles (figure 2) and flow structure. This bangle also presented abundant white spherulites identified by Raman spectroscopy as cristobalite (a polymorph of silica). These spherulites were small and individually dispersed throughout the glass (figure 3). The presence of this high-temperature low-pressure silica phase indicates a natural high-temperature formation. From all the gemological, FTIR, and Raman data collected, this bangle was identified as Libyan Desert glass. Most Libyan Desert glass is tubular in shape, and some are approximately spherical or rod-like (R.A. Weeks et al., “Libyan Desert glass: A review,” Journal of Non-Crystalline Solids, Vol. 67, No. 1-3, 1984, pp. 593–619). Those fragments are usually 2 to 63 mm in size, so it is surprising to examine this material in the form of a bangle bracelet.

gems, jade, dickite, polymer, plastic dickite imitation

The authors recently encountered a yellow carving (Figure 27) resembling dickite or perhaps talc, such as documented for another piece from the same collec- tion (Tay et al. 2023). However, this one turned out to be much different. When handled, the 3,550.6 g carving seemed rather heavy for its size and had a plastic-like feel. When examined with a 10× loupe, small rounded beads were visible along grooves in the carving (Figure 28), and some parts showed specks of white inclusions. The spot RI reading was 1.55, and unlike dickite, the carving fluoresced weak chalky blue to a long-wave UV lamp (although it was inert to short-wave UV radiation).

gems, spinel, inclusions, gemological characteristics, Raman spectrum, Myanmar, Mogok, Tajikistan

Currently, most of the popular spinels in the jewellery market come from Myanmar and Tajikistan. It is well known that provenance is one of the main factors affecting the value of a gemstone, and the geographic origin of a gemstone can be determined by examining its gemological and inclusion characteristics. This study systematically characterized the conventional gemology of spinels from Myanmar and Tajikistan and compared the inclusions in the spinels from these two countries by means of gemological microscopy and Raman spectroscopy. The results showed that most red and pink Myanmarese spinels were octahedral or contact twins, while Tajikistan spinels are slabbed or octahedral distorted crystals. Columnar zircon is frequently found in Tajikistani spinels but rare in Myanmar spinels, appearing as tiny accessory inclusions. There are three types of carbonate inclusions (magnesite, dolomite, and calcite) in Myanmar spinels, but Tajikistan spinels have only one (magnesite). In addition, spinels of different origins include special inclusions. Myanmar spinels contain pyrite inclusions; Tajikistan spinels contain rutile and talc inclusions.

Primary nephrite deposit, Meteoric water, Zircon U–Pb dating, Chuncheon, South Korea, gems, jade, nephrite

The Chuncheon nephrite deposit is the largest in South Korea with an estimated reserve of 300,000 tons. Located in mid-eastern South Korea, the deposit is in the form of lenses in the contact zone between dolomitic marble and amphibole schist in the upper part of a biotite schist. To date, the origin, process of formation, and age of the Chuncheon nephrite deposit remain unclear, which hinders our further understanding of the formation of nephrite. This study involved petrography, chemical analyses, and nephrite age determinations using optical mineralogy, electron microprobe analyses (EMPA), X-ray fluorescence spectroscopy (XRF), and SHRIMP (Sensitive High-Resolution Ion MicroProbe) analyses of zircon. Petrographic studies indicated that the Chuncheon nephrite consists mainly of tremolite. The low contents of Cr (2.15–4.74 ppm) and Ni (2.95–4.90 ppm) are typical of dolomite-related nephrites and lower than in serpentine-related nephrites. The total REE contents are <18.5 ppm. The REE chondrite-normalized patterns display right-leaning LREE and almost flat HREE patterns, which are consistent with the REE model of dolomitic marble. The δ¹⁸O isotope compositions of the zircons (6.34‰–12.5‰) from the nephrite overlap those of the Chuncheon dolomite (2.4‰–18.2‰), indicating clearly that dolomite played an important role in the formation of the Chuncheon nephrite. The petrography and geochemistry suggest that it had a metasomatic origin involving the reactions dolomitic marble → diopside, diopside → tremolite, and tremolite → chlorite. The δD and δ¹⁸O isotope compositions of our specimens of nephrite (δD = −95.3‰ to −78.3‰ and −118‰ to −105‰; δ¹⁸O = −9.9‰ to −7.9‰) and the hydrothermal fluids in equilibrium with the nephrite (δD = −73.6‰ to −56.6‰ and −96.3‰ to −83.3‰, 330–450 °C; δ¹⁸O = −9.41‰ to −6.38‰, 330–430 °C) suggest that the hydrothermal fluids were predominantly meteoric water. The zircons contain almost no mineral inclusions, which is quite different from zircons in other nephrite deposits. The variable SHRIMP U–Pb ages of the zircons (908.9 ± 4.8 to 459.1 ± 1.9 Ma (n = 7), 436.3 ± 16.9 to 359.5 ± 6.4 Ma (n = 5), and 249.9 ± 5.2 to 79.4 ± 1.9 Ma (n = 5)) in the Chuncheon nephrite may record the ages of a regional multi-stage metamorphism. Due to the generally low temperature of formation of the nephrite, we speculate that the ages of zircons only record the two relatively high-temperature initial stages of nephrite formation (dolomitic marble → diopside, and diopside → coarse-grained tremolite) and failed to record the age of the lowest-temperature third stage of high-quality nephrite formation (coarse-grained tremolite → fine-grained tremolite). It has generally been considered that dolomite-related nephrite forms at the contacts between granite/granodiorite and dolomitic marble, whereas serpentinite-related nephrite forms at the contacts between serpentinite and silica-rich rocks. However, the genesis of the Chuncheon nephrite deposit does not fit with either of these well-known types. Thus, we developed a new model for the Chuncheon nephrite in which heat was provided by regional multi-stage metamorphism, meteoric water dominated the nephrite-forming fluids, and dolomitic marble and biotite schist provided the material for the nephrite.

gems, corundum, inclusions.5, China.12, sapphire, Changle, Shandong