gems, spectroscopy, advanced instrumentation, Raman specrometry, gemology

The current paper gives an overview of the development of Raman spectrometry in gemmological laboratories. While before 1990s, no commercial gemmological laboratory possessed such an instrument, all larger international labs have acquired these instruments by now. The Raman spectrometer is routinely used for the detection of emerald fillers, HPHT treatment of diamonds, analysis of the nature of a gemstone, analysis of gemstone inclusions and treatments, and the characterisation of natural or colour enhanced pearls and corals. Future developments in gemstone research lie in the closer analysis of the features of Raman and PL spectra and in the combination of several instruments.

jade, Nephrite, Placer nephrite, Geochemistry, Khotan, Hetian, Xinjiang, Northwest China, gems

The Yurungkash (White Jade) and Karakash (Black Jade) rivers in Hetian, Xinjiang Province, are China’s principal sources of white, green, and black placer nephrite, with a history of mining spanning ~5000 years. Twenty-nine nephrite samples collected from these rivers show fine-grained, compact microstructures. Mineral assemblages indicate formation via metamorphic/metasomatic reactions: dolomitic marble → tremolite, or dolomitic marble → diopside → tremolite. White nephrite (Yurungkash) and green nephrite (Karakash) are mainly tremolite. Two types of black nephrite were identified: actinolite aggregates, and tremolite with graphite crystals up to 2 mm. Compared to black nephrite, white and green varieties show fewer inclusions and lower FeO and MnO content. Amphiboles have very low Cr₂O₃ (0.00–0.07 wt.%) and NiO (0.00–0.05 wt.%), much lower than serpentinite-related nephrites. Total REE contents are generally low (12.22–49.40 ppm), with a few higher cases due to REE-bearing minerals. Chondrite-normalized REE patterns show strong negative Eu anomalies (0.16–0.48), moderate LREE enrichment (La/Ndₙ = 1.8–5.0), and flat HREE distributions (Gd/Ybₙ = 0.3–1.7). Stable isotope values (δ¹⁸O = 1.1‰–5.6‰; δD = −55.7‰ to −72.4‰) match those of dolomite-related nephrites and are consistent with magmatic water at 330–430 °C. These findings confirm that the placer nephrites are of dolomite-related metamorphic/metasomatic origin, derived from primary nephrite deposits in the nearby Kunlun Mountains.

corundum, color.4, Afghanistan.12, gems

In March 2010, the Bangkok laboratory received a rough parcel of 175 blue sapphires reportedly from a relatively new deposit near the lapis lazuli mines in the Kokcha Valley of Afghanistan’s Badakhshan Province. The pieces ranged from ~0.2 to 4 g, and about half the parcel was gem quality. During a trip to Afghanistan in July 2010, one of us (VP) obtained six additional samples said to be from this deposit, consisting of blue sapphires on a matrix of bluish green and yellowish brown crystals (identified as dravite and phlogopite by Raman and laser ablation–inductively coupled plasma–mass spectrometry [LA-ICP-MS] analysis, respectively). Of the 181 samples, most had strong hexagonal blue color zoning surrounding a yellow core that displayed a six-rayed star pattern when viewed with transmitted light. Common inclusions were negative crystals (sometimes associated with healed fissures), corroded zircon crystals, twin planes, and crystals of corundum. Most of the sapphires were sector zoned, with colorless areas associated with the basal pinacoid.

corundum, treatments.6, gems

Trace amounts of natural beryllium are sometimes detected in untreated sapphires, such as those from Madagascar, typically ranging from <1 ppma to about 4–5 ppma Be, while higher concentrations (>15 ppma) are usually associated with beryllium diffusion treatment. Naturally occurring beryllium in corundum is often accompanied by traces (usually <1 ppma) of niobium, tantalum, light rare-earth elements, and thorium (see A. Shen et al., “From the GIA Laboratory: Beryllium in corundum: The consequences for blue sapphire,” GIA Insider, Vol. 9, No. 2, Jan. 26, 2007). During routine testing, three blue sapphires recently submitted for identification were found to contain relatively high levels of beryllium; the three consisted of one ring-mounted stone and two loose sapphires weighing 3.17 and 3.21 ct. Two samples showed microscopic evidence of heating, while the third (3.17 ct) had inclusion features proving it was unheated. The 3.21 ct sapphire showed a chalky blue reaction to short-wave UV (consistent with low iron content) and strong blue bands in immersion, whereas the other two were inert to long- and short-wave UV due to higher iron contents. All three contained milky clouds. LA-ICP-MS trace-element analysis revealed Be concentrations ranging from virtually absent to 33 ppma, generally positively correlated with transition metals and light rare-earth elements including niobium, tantalum, tungsten, lanthanum, cerium, hafnium, and thorium. These higher concentrations were often associated with clouds, as seen in Be-bearing untreated sapphires. Magnesium also showed a positive correlation with Be, albeit with wide variation, while titanium displayed both positive and negative correlation, possibly due to its zoning in blue areas. The Be concentrations in these sapphires were comparable to or exceeded those in some Be-diffused stones. Their properties suggest origins from at least two distinct deposits or deposit types, though specific sources could not be determined. This case reinforces the necessity of thorough gemological and chemical analysis to identify beryllium-diffusion treatment.

gems, glyptic, inclusions, garnet, moonstone, quartz

The gemstone sources that supplied raw materials to ancient gem cutters are known to us today mostly from ancient inscriptions and texts. Many sources are described, and clearly more varieties of gems were known than have been identified positively or have survived to the present day. Together, texts and archaeological work sometimes have revealed associations between extant carved gems and gem-producing localities, despite the paucity of direct evidence of gem-mining activity. In addition, archaeology has rectified written accounts that sometimes confuse centres of gem industry with gems at their sources. It also has provided illuminating details about gem production and the circulation of gems throughout the ancient world – whether raw materials, partially worked blanks, or finished, fashioned gems discovered in workshops, trading centres, or settlement and burial contexts.

gems, spinel, Afghanistan, Tajikistan, balas ruby, Kuh-i-Lal, Pamir, geology, inclusions, fluid inclusions

The Kuhilal deposit is located in the Ishkashim region of the Gorno-Badakhshan Autonomous Region (Southwestern Pamir). It has been known since ancient times (7th century) as the source of the famous Badakhshan Lal (noble spinel). It is localized in the lower structural-stratigraphic unit of the metamorphic sequence of the Southwestern Pamir—the Goransky series. In this series, the bulk of the sediments is represented by biotite and biotite-hornblende gneisses and migmatites that contain multiple bands, layers and lenses of magnesite marbles. Structurally, the deposit is located on the western limb of the Garmchashminsky anticline, complicated by folds of lower orders and by a series of small, compressed and often overturned isoclinal folds. The deposit is a unique occurrence that combines distinctive mineralogy, a wide range of minerals and unique features of genesis. Various types of non-metallic mineral raw materials of industrial significance are concentrated here.

gems, jade, jadeite, fei cui, geology, inclusions

Jadeitites form from hydrothermal fluids during high-pressure metamorphism in subduction environments; however, the origin of zircons in jadeitite is uncertain. We report ion microprobe analyses of δ¹⁸O and Ti in zircons, and bulk δ¹⁸O data for the jadeitite whole-rock from four terranes: Osayama serpentinite mélange, Japan; Syros mélange, Greece; the Motagua Fault zone, Guatemala; and the Franciscan Complex, California. In the Osayama jadeitite, two texturally contrasting groups of zircons are identified by cathodoluminescence and are distinct in δ¹⁸O: featureless or weakly zoned zircons with δ¹⁸O = 3.8 ± 0.6‰ (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher δ¹⁸O = 5.0 ± 0.4‰. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar δ¹⁸O values to the latter from Osayama: Guatemala zircons are 4.8 ± 0.7‰, and the Syros zircons are 5.2 ± 0.5‰ in jadeitite and 5.2 ± 0.4‰ in associated omphacitite, glaucophanite, and chlorite-actinolite rinds. The δ¹⁸O values for most zircons above fall within the range measured by ion microprobe in igneous zircons from oxide gabbros and plagiogranites in modern ocean crust (5.3 ± 0.8‰) and measured in bulk by laser fluorination of zircons in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6‰). Titanium concentrations in these zircons vary between 1 and 19 ppm, within the range for igneous zircons worldwide. Values of δ¹⁸O (whole-rock) and δ¹⁸O (jadeite) vary from 6.3 to 10.1‰ in jadeitites in all four areas. These values of δ¹⁸O and Ti are higher than predicted for hydrothermal zircons, and the δ¹⁸O values of most zircons are not equilibrated with the coexisting jadeite at reasonable metamorphic temperatures. We conclude that while some zircons may be hydrothermal in origin, a majority of the zircons studied are best explained as relic igneous crystals inherited from precursor rocks; they were not precipitated directly from hot aqueous fluids as previously assumed. Therefore, U–Pb ages from these zircons may date magmatic crystallization and do not establish the timing of high-pressure metamorphism or hydrothermal activity.

gems, jade, jadeite, fei cui, Jadeitite, Zircon, Serpentinization, SHRIMP-RG, Guatemala

Zircons from one jadeitite sample, collected from serpentinite mélange north of the Motagua fault, Guatemala, were separated for SHRIMP-RG U–Pb dating and trace element analyses. The sub- to anhedral crystal form, lack of typical magmatic oscillatory zoning, the presence of fluid and albite/quartz/jadeite inclusions, and the low Th/U ratios (<0.005) indicate that these zircons would have precipitated from an aqueous solution. The U–Pb concordia intercept age of these zircons is 95.1 ± 3.6 Ma, much older than the reported ⁴⁰Ar/³⁹Ar phengite age (i.e., 77–65 Ma) of jadeitites and related rocks from the same area. The 95 Ma age is interpreted as the time of jadeitite formation, whereas the 77–65 Ma age reflects the time of cooling after isotopic resetting due to superimposed metamorphism at ∼70 Ma. The superimposed metamorphic temperature would therefore be higher than the phengite ⁴⁰Ar/³⁹Ar blocking temperature and probably at 400–450 °C. It is further concluded that “metasomatic replacement” and “vein precipitation” are two end-member mechanisms for jadeitite genesis. Both the whole-rock jadeitite and its zircons have very low REE contents, ∼1 ppm and 0.5–42 ppm, respectively. The whole-rock jadeitite shows a flat and slightly concave REE pattern and a positive Eu anomaly (Eu/Eu* = 1.24). Zircons are enriched in HREE and their REE patterns can be divided into two groups: one with negative Eu anomaly and one with positive Eu anomaly. The latter tends to have smaller positive Ce anomalies. The fluid from which jadeitite formed probably evolved over time, becoming more reducing and more dominated by plagioclase decomposition reactions. Alternatively, trace element compositions of zircons simply demonstrate complicated variations of fluid chemistry during jadeitite formation. A reducing fluid with high pH values capable of mobilizing Al, Na, Zr and Hf is inferred to be the media during jadeite/zircon formation. If jadeitite formation was related to serpentinization fluid, the ultramafics must contain olivine as the major primary phase subjected to serpentinization, yielding reducing fluids with high pH values. Two additional requirements are (1) the presence of feldspar/mica-bearing protolithic blocks survived from rodingitization in peridotites to provide Al and Na, and (2) the presence of fluid channels as well as pulses of in/out-flux fluid to trigger metasomatic reactions. These preconditions in subduction zones may contribute to the rare occurrences of jadeitite in global high-P metamorphic belts.

gems, corundum, inclusions.5, Burma.12, Sri Lanka.12, Madagascar.12, Tanzania

Apatites derived from different rock types have distinctive absolute and relative abundances of trace elements including rare-earth elements (REE). [1] In this study, apatite inclusions in natural gem-quality corundum and spinel were analysed. They originate from the world’s most important mining areas including Myanmar, Vietnam, Sri Lanka, Madagascar and Africa. Bulk compositions of the apatite inclusions and the host-mineral were determined using LA-ICP-MS. Differences in LREE/HREE fractionation and Eu-anomalies in apatite were found. They can be compared with the variations of trace and minor element concentrations in the host-corundum and spinel. There is no significant difference in total REE concentration between apatites in corundum and in spinel. The most interesting observation is that only samples from Winza (Tanzania) show a positive Eu-anomaly (Fig.1), which was already found in pargasite and garnet inclusions of the same provenance. [2] All the other samples showed a negative Eu- anomaly or no anomaly. No anomaly was found in two samples from a new ruby mine in Mozambique.

corundum, Vietnam.12, basaltic volcanism, continental Earth’s crust, sapphire, zircon, Vietnam, gems

Study of the chemical composition of clinopyroxene and garnet megacrysts from the Dak Nong sapphire deposit, along with model calculations, indicates that the megacrysts originated from the crystallization of alkali basaltoid magma in a deep-seated intermediate chamber at pressures of 14–15 kbar, corresponding to a depth near the Moho (~50 km) in southeastern Asia. This chamber served as a heat and CO₂ fluid source for generating crustal syenitic melts that produced sapphires and zircons. The formation conditions of these minerals differ significantly: jadeite inclusions in placer zircons indicate high-pressure crystallization, corroborated by decrepitation of CO₂-rich melt inclusions, whereas sapphires formed from iron-rich syenitic melt in shallower crustal levels, influenced by CO₂ and carbonate–H₂O–CO₂ fluids. Subsequent eruptions of alkali basalts transported garnet and pyroxene megacrysts, along with sapphire and zircon xenocrysts, to the surface. The study concludes that sapphire deposits can form only during multistage basaltic volcanism involving deep intermediate chambers in areas with thick continental crust. The presence of a widespread megacryst assemblage—clinopyroxene, garnet, sanidine, and ilmenite—and placer zircon megacrysts may serve as useful indicators in sapphire exploration.