Gem Localities and Formation
Scientific Study of Colored Gem Deposits and Modern Fingerprinting Methods
Lee Groat
University of British Columbia, Vancouver, Canada
Most colored gemstones form near the earth’s surface in a wide range of different environments; for example, they can crystallize from igneous magmas or hydrothermal solutions, or via the recrystallization of preexisting minerals during metamorphism. The specific environment determines the types of gem minerals that form, as well as their physical and chemical properties. Field studies of colored gem deposits provide the basis for the scientific understanding of natural gemstone formation and, in turn, the basis for criteria for gem identification.
Gem deposits are of scientific interest because they represent unusual geologic and geochemical conditions; for example, emeralds are rare because they require beryllium and chromium (and/or vanadium), which generally travel in very different geochemical circles. Scientists study gem deposits by collecting rock and mineral samples in the field, mapping geological formations and structures, documenting the environment in which the gems occur, and examining the collected samples back in the laboratory. Such examination yields information on the chemical, temperature, and pressure conditions of gem formation, the associated minerals (often found as distinctive inclusions in the gems themselves), and the age of the deposit. Determining the origin of a gem deposit usually requires a small amount of very specific data. The results are published in publicly available peer-reviewed publications. Such field studies provide clues that can be used to explore for similar types of gem deposits. Challenges include the remoteness of locations that have not been previously studied by geologists, the small size of deposits that precludes study by large mining companies, and the rarity of the gems themselves.
There is much left to do in gem deposit research. For example, despite its growing popularity as a gemstone, there are few studies of gem spinel deposits, especially cobalt-blue spinel (figure 1), for which only one deposit has been studied. To date we know little about what factors control spinel genesis and color.
Recently there has been another reason to study gem deposits: gem fingerprinting, in which modern methods are used to obtain characteristic information. This information is then compared to information obtained from stones from known localities to estimate where a stone with no locality information originated.
Modern fingerprinting methods analyze the chemistry of the stones (using electron probe microanalysis, isotopic analysis, laser ablation–inductively coupled plasma–mass spectrometry) and/or their solid and fluid inclusions. We know that the chemistry of the stones must reflect the chemistry of the host rock environment; for example, the chromophore in emerald from Lened in Canada is vanadium, and not the typical chromium, because there are no chromium-bearing rocks in the area. With respect to solid inclusions, rubies from Aappaluttoq in Greenland have phlogopite mica inclusions because they recrystallized in a rock at pressures and temperatures where phlogopite is the stable potassium-bearing phase. An example of diagnostic fluid inclusions is the three-phase variety seen in Colombian emeralds (and now also observed elsewhere). New is the use of ICP-MS on fluid inclusions to define part of the fluid assemblage from which the stones were formed; this tells us about the environment of formation, but also may assist in defining a fingerprint for the stone.
Where scientific studies require only very specific data, the more data available from stones of known origin, and the more representative those stones are of the full range of compositions and inclusions found in a specific deposit or country of origin, the more accurate the estimation should be. Unfortunately, these data are generally not made public, so every lab doing fingerprinting is essentially working independently, and there is no way to know how accurate their data and the resulting country- or deposit-of-origin estimates are. We also note that a serious problem in origin determination is that some of the best gemstones will be lacking diagnostic inclusions altogether, which then restricts the tools and observations can be used.
Gem Pegmatites of Ukraine, Russia, Afghanistan, and Pakistan: An Update on Recent World-Class Finds
Peter Lyckberg
National Museum of Natural History, Luxembourg
Soviet-era exploration and mining of some 1,900 chamber pegmatites on the western endo-contact of the 1.7 Ga Korosten Pluton in Ukraine for piezo quartz left an underground mine with 110 km tunnels reached by six shafts. Since 1995, increasingly intensified study of old pockets and documentation by Vsevolod Chournousenko, chief geologist of Volhyn Kvarts (Quartz) Samotsvety Company, has revealed new targets. Work on these targets has produced gem-quality beryl and topaz, the latter in record-size crystals up to 230 and 325 kg. Topaz occurred in around 10% of mined pockets.
For the first time in the history of the huge deposit, the “Peter’s Dream Pocket” was found and exploited from January 2013 through January 2014. It contained bicolor topaz still in situ growing on cleavelandite, associated with zinnwaldite and smoky quartz. The largest topaz, at 325 kg, was named dedushka (grandfather). The Skorkina pocket in the mine was also called “Vsevolod’s Pocket” after the extraordinary chief geologist who found the topaz mineralization, which yielded a record amount of gem topaz crystals. These came from a depth of 6 to 15 m under the floor, where previous Soviet quartz mining efforts had missed them. The largest of these, the cognac-colored “Sergei” (named after the mine owner), weighs 230 kg and is the largest of its kind. Gem-quality bicolor topaz in large sizes, found particularly in shaft 3 and in open pits in the south and north end of the pegmatite field, is unique to this mine. Extraordinary heliodor crystals were mined in Soviet times, and a smaller quantity mined later yielded faceted stones up to 2,500 ct. A recently mined deep cognac-colored cut topaz weighed 9,000 ct. Natural blue and bicolor topaz (figure 1) have been cut to unique stones, also of large sizes. The color spectrum of topaz and beryl from these pegmatites is amazing.
Lyckberg et al. (2009) noted that gem beryl occurs in only 2% of the approximately 1,900 chamber pegmatites that were mined. Pegmatite 521 at 90 m depth, accessed from shaft 2, produced over two tons of gem beryl. Over one ton was exploited in 1982 and in 1992, over the course of five days, five miners excavated 900 kg in a green clay zone. Ninety percent of these specimens were of gem quality. In the same pegmatite, a second pocket produced 100 tons of quartz in the 1980s. During the spring of 2018, exploration of this pocket yielded a giant quartz crystal measuring 1.5 m in diameter. The pocket was found to also contain topaz pseudomorphs.
In neighboring Russia, gem aquamarine was produced in 2017 at Sherlova Mountain and sold to China, while in the Ural Mountains only small quantities for collectors were found. Here the last Russian underground gem pegmatite mine, the Kazionnitsa, closed in 1993. In the Malkhansk Mountains, tourmaline-rich lithium pegmatites with quality rubellite crystals were recovered during 2012–2018 at the Sosedka pegmatite. They were a deep red to purplish red color in crystals up to 35 cm in matrix, while gem-quality specimens measured up to 10–15 cm. Many of these crystals are reminiscent of the rubellite from the Jonas mine in Brazil, although the cranberry red color is not quite as intense. Several other pegmatite veins started to produce again, primarily green tourmaline.
The Hindu Kush pegmatites of Afghanistan discovered at Kala and Paprok villages in 1959 and 1969 have again yielded large quantities of gem tourmaline in a rainbow of colors. The huge Mawi and Kanakana pegmatites both produced large quantities of gem kunzite, gem indicolite, and rare morganites. Gem-quality pollucite and other rare species such as manganotantalite were found at Paprok and in the Pech Valley. Kanakana and several other pegmatites produced large morganites with aquamarine cores in crystals up to 25 cm in diameter growing on lepidolite and cleavelandite. The pegmatite field of Waygal produced perhaps the finest single 7 kg kunzite crystal of any find, a flawless 55 cm twinned deep purple crystal with blue 10 cm termination in a pocket with 10 kg other gem-quality crystals.
The Karakorum Mountains of Pakistan have continued to produce large quantities of aquamarine and champagne-colored topaz. Please note that beryl and topaz in these very young 4 to 12 Ma pegmatites have not yet been exposed to radiation from K40 in the feldspars or from U/Th-containing minerals during this short time span. Thus, deeply colored yellow heliodor, blue topaz, and deep orange topaz are not to be expected here, simply because they have not had a chance to attain those colors (Unpublished data by the author, 1997). Those colors are typical for pegmatites one or two magnitudes older.
Gem-quality rare species mined include large colorless to light lilac gem pollucite (10–40 cm), amblygonite, manganotantalite, various microlites, triplite, green hydroxylherderite, beryllonite in cogwheel crystals up to 35 cm, and väyrynenite in orange-red gem crystals up to 22 cm long. The area around Shengus and Bulochi, by the Indus River between Nanga Parbat and Haramosh Peak, is the main producer of the rarer species. Since 1985, known hydrothermal mica-lined fissures at Chumar Bakhoor (Unpublished data by the author, 1988; Lyckberg et al., 2013) have been a major source of matrix aquamarine specimens, large crystals for carvings, and much cabochon and bead material, as well as large pink and green gem-quality fluorite, some of which have been faceted.
REFERENCES
Lyckberg P., Chournousenko V.A., Wilson W.E. (2009) Famous mineral localities: Volodarsk-Volhynsk, Zhitomit Oblast, Ukraine. The Mineralogical Record, Vol. 13, No. 6, pp. 11–22.
Lyckberg P. Chournousenko V., Hmyz A. (2013) Chamber pegmatites of Volodarsk, Ukraine, the Karelia beryl mine, Finland and shallow depth vein pegmatites of the Hindukush-Karakorum mountain ranges. Some observations on formation, inner structures, rare and gem crystals in these oldest and youngest pocket carrying gem pegmatites on Earth. In Contributions to the 6th International Symposium on Granitic Pegmatites, pp. 81–83, http://pegmatology.uno.edu/news_files/PEG2013_Abstract_Volume.pdf
A Multidisciplinary Approach Toward Examining the Sources of Emeralds
Raquel Alonso-Perez1, Adriana Heimann-Rios2, James M.D. Day3, Daniel X. Gray2, Antonio Lanzirotti4, Darby D. Dyar5, and J.C. “Hanco” Zwaan6
1Harvard University, Cambridge, Massachusetts
2East Carolina University, Greenville, North Carolina
3Scripps Institution of Oceanography, La Jolla, California
4The University of Chicago, Argonne, Illinois
5Mount Holyoke College, South Hadley, Massachusetts
6Naturalis Biodiversity Center (National Museum of Natural History), Leiden, The Netherlands
Modern analytical capabilities now allow the combination of nondestructive geochemical and structural studies of emeralds, in addition to detailed studies of their inclusions, to enhance our knowledge of their genesis. Here we present a combination of (1) X-ray absorption near-edge structure (XANES), to determine local coordination environment and oxidation state of the main emerald chromophores Fe, V, and Cr; (2) Raman spectroscopy, with special emphasis on the correlation between H2O molecules and alkali site occupancy; and (3) inductively coupled plasma–mass spectrometry (solution-ICP-MS) to examine the role of major, minor, and trace elements during emerald formation. Our aim is to develop a systematic approach to characterizing emeralds by identifying key geochemical and structural features that enable provenance and geological origin of emerald deposits to be determined.
In this study, we analyzed 31 emeralds from the Mineralogical & Geological Museum, Harvard University. Preliminary XANES results indicate that chromium is present as Cr3+, although crystal orientation dependence and beryl crystal structural behaviors need to be assessed in detail. Raman spectroscopy results of the OH-stretching vibrations at higher frequencies (3500–3700 cm–1), corresponding to H2O type II and type I, respectively, display an orientation dependence. For a given orientation, there is an increase in intensity of the OH-stretching vibration, and therefore H2O concentration, from Colombian to Zambian emeralds. A strong correlation of peak shape and position of the OH-stretching vibration with three major geochemical indices is also observed. Vanadium concentrations correlate positively with Ge, Rb, Cs, and Lu, and they can be used to distinguish three emerald groups based on their geochemistry: (1) Colombia; (2) South Africa, Nigeria, and Egypt; and (3) Brazil, Madagascar, and Zambia. Whereas a smooth transition occurs from group 2 to group 3, group 1 Colombian emeralds are highly distinctive. The distinctiveness of Colombian emerald indicates the potential for using trace-element abundances to examine geological formation (see figure 1). For example, ratios such as Ga/Rb versus Hf/Ta are also characteristic of each different emerald formation. The combination of XANES, Raman spectroscopy, and ICP-MS studies offers significant utility for not only refining the crystal structure of emeralds but also defining markers for different sources.
REFERENCE
Rudnick R.L, Gao S. (2004) Composition of the continental crust. In H.D. Holland and K.K. Turekian, Eds., Treatise on Geochemistry. Vol. 3, The Crust, Elsevier-Pergamon, Oxford, UK, pp. 1–64.
The Proof of Provenance
Klemens Link
Gübelin Gem Lab, Lucerne, Switzerland
Consumers now demand more transparency on the provenance of the goods and services they purchase, an expectation that also applies to luxury products such as gemstones. Yet the gemstone trade has a reputation of being opaque. The Gübelin Gem Lab has kicked off a long-term initiative dedicated to closing this gap. Under the label Provenance Proof, technologies are developed that allow gemstones to be traced back to specific mines in order to provide more transparency throughout the chain of custody. The Provenance Proof initiative is independent of the traditional lab work, and all technologies are made available to the entire industry.
The first technology developed under the Provenance Proof initiative is referred to as the Emerald Paternity Test. The Gübelin Gem Lab has developed a physical tracer using nanolabels to suit the needs of the gemstone industry. These nanolabels are based on DNA fragments encapsulated in silica; with a diameter of 100 nm, they are invisible even to the most powerful optical microscope. The identity of the mine and the miner, the exact location, the time period, and possibly further information are encoded into the DNA. The use of such labels allows the miner to tap the full potential of storytelling about specific provenance, including the possibility of independent proof of this source.
The nanotracers allow the tracing of a gemstone from the end consumer back to the original mine. Nevertheless, the other steps along the value chain remain nontransparent. To shed light on the entire value chain, we developed a digital logbook that allows the user to document all steps and transactions linked to a particular gemstone on its way from the mine to the end consumer. The backbone of this logbook is a hyperledger-based, community-controlled blockchain that is open to the entire industry and accessible with a smartphone. All data is securely encrypted and decentralized and only available to the holder or custodian of a specific gemstone.
