gems, corundum, treatments.4, heat treatment detection, FTIR, 3232

Describes the appearance of a 3232 FTIR peak in a pink sapphire from Madagascar that appeared to be unheated. This peak is typically associated with gems that have been heat treated.

gems, corundum, ruby, Pakistan.12, assembled stones.8, beryl, emerald, assembled specimens

Recent field observations in Pakistan have revealed that specimens circulating in local gem markets and near mine sites have increasingly been artificially assembled or altered. Local dealers and prospectors from the Hunza and Gilgit mining areas offered “Frankenstein” specimens, pieces assembled by gluing together fragments of crystals and host rock (figure 6). While adhesives can sometimes be seen under magnification, many joints are subtle enough to mislead buyers or tourists. Crystals such as aquamarine are often skillfully mounted onto matrices to imitate natural specimens, with some pieces further polished, dyed, or even oiled to enhance their appearance (figure 7). In certain cases, resins mixed with crushed marble or host rock are applied, making detection even more difficult. For example, in Peshawar’s Namak Mandi gem market, a ruby reportedly from Jegdalek was identified as a synthetic crystal mounted on natural host rock. In addition to this deceptive practice of adhering specimens to host rock, broken specimens are also repaired with adhesives, often without disclosure. With time, the quality of assembly is improving, making detection more challenging. The close proximity of these reconstructed specimens to the mines is a reminder that “from the mine” does not automatically guarantee authenticity. Collectors and gemologists should be aware that stones described as directly sourced may have undergone significant human intervention. Similar practices are common in neighboring regions, including Afghanistan.

gems, corundum, geology.11, Burma.12, Mogok, beryl, emerald, Russia, Malysheva, Ural Mountains, alexandrite, chrysoberyl, pegmatite

It is widely recognized that some gem minerals often occur together in various localities—either in the same host rock or in the same deposit. Gem minerals each require a certain set of physical and chemical conditions for their formation. This edition of the Colored Stones Unearthed column will explore where specific gem minerals are found together and the conditions that produce these distinct geological settings.

gems, corundum, color.4

The 3161 cm⁻¹ infrared (IR) band can be observed in variously colored natural corundum, most commonly in yellow sapphire. The presence of this feature is used frequently as supporting evidence to conclude that a sapphire has not been heat treated (C. M. Breeding and N. J. Ahline, “Infrared spectroscopy and its use in gemology,” Winter 2024 G&G, pp. 474–492). This 3161 cm⁻¹ band is often present with other less intense side bands known as the “3161 cm⁻¹ series” and includes features at ~3355, 3242, 3100 (shoulder), 2460, and 2420 cm⁻¹ (C. P. Smith and C. van der Bogert, “Infrared spectra of gem corundum,” Fall 2006 G&G, pp. 92–93). The exact origin of the 3161 cm⁻¹ band is not well understood, although it is thought to be related to stretching vibrations of hydroxyl groups caused by interstitial H⁺ ions present as charge compensation for Fe²⁺ substituting for Al³⁺ in the corundum structure (M. C. Jollands et al., “Vibrational properties of OH groups associated with divalent cations in corundum (α-Al₂O₃),” European Journal of Mineralogy, Vol. 35, No. 5, 2023, pp. 873–890).

gems, corundum, origin determination

Determining the geographic origin of ruby and sapphire is vital for their valuation, authentication, and ethical sourcing, yet traditional methods relying on expert assessment of visual, chemical, and spectroscopic traits can be subjective and inconclusive when deposits share overlapping features. This study applies machine learning to trace-element geochemical data from rubies and sapphires analyzed by EDXRF and LA-ICP-MS. Models trained on LA-ICP-MS data, with its superior sensitivity, achieved markedly higher classification accuracy than those based on EDXRF. Among the algorithms tested, random forests and artificial neural networks exceeded 90% accuracy for both ruby and sapphire, though metamorphic sapphires remained the most difficult to classify due to overlapping chemical profiles. The findings highlight machine learning’s potential to revolutionize gemmology by providing a scalable, reproducible, and objective framework for origin determination, paving the way for more standardized gem certification.

gems, corundum, Tanzania.12, Umba, East Africa, ruby, sapphire, fancy sapphire

During the late 1980s, the first author Terrence Coldham (TC) purchased a tin trunk filled with rocks and rough gem materials from Orestis Palos, at the time the owner of a Sydney-based gemstone wholesaler trading as Gempalos Co. Although Orestis rarely spoke about his life before arriving in Australia, he did mention spending time in Tanzania, and that was the origin of the materials in the trunk. Given author TC’s long-standing interest in corundum, the contents were immediately fascinating: coarse-grained rocks containing corundum crystals, along with many loose pieces of rough sapphire in a wide range of pastel colours. Orestis explained that all the material was from Tanzania, and aside from the ruby in zoisite, everything had come from the Umba region. Soon after acquiring the rough material, several stones were selected to be cut and polished by well-respected Sydney lapidary Tony Maynard, resulting in a collection of faceted and cabochon Umba sapphires which are still in author TC’s possession. Despite having visited Tanzania several times over the years, TC had never managed to reach the Umba gem fields, the very source of the Umba sapphire collection. That changed in October 2024 when two of the authors (TC and SS) travelled to Tanzania, specifically arranging a visit to Umba.

gems, advanced testing, cathodoluminescence, CL

Recently, the author examined a faceted blue oval sapphire. Standard gemological properties and chemical analysis indicated a geographic origin of Kashmir. Microscopic examination revealed internal graining, fine particle banding, and fingerprint inclusions. Interestingly, elongated zircon inclusions were also observed, displaying attractive birefringent interference colors under cross-polarized filters caused by the splitting of light in the doubly refractive crystals. Kashmir sapphire is one of the rarest and most valuable gemstones in the world, renowned for its beauty and quality, and this stone was no exception. [RWH: With brief notes like this, one should not place too much confidence in the origin determination]

gems, ancient gems, corundum, treatments.6

The history of heat treatment falls within an interdisciplinary field of inquiry, involving the reconstruction of know-how from archaeological discoveries; drawing on pre-scientific written sources, when available—chiefly lapidaries, alchemical treatises, and technical recipe books; experimenting with the methods described in those texts; and conducting gemological analyses to determine whether the gemstones have been heat-treated. Until now, experiments and analyses have mainly been carried out in the archaeological sphere, primarily in prehistoric and protohistoric contexts, as part of research into the lithic industry. In contrast, this question has been scarcely addressed for later periods such as the Middle Ages and the modern era (5th century to the end of the 18th century). It is important to emphasize that in ancient texts, one finds both practices with actual effectiveness and magical practices rooted in belief and ritual—such as the use of goat’s blood to cut diamond, first mentioned by Pliny the Elder in Natural History (Book 37, Chapter 15, Section 3) in the 1st century, and later repeated in treatises on gems, even during the Renaissance. Furthermore, reconstructing these recipes is not always easy, as those who wrote them were not necessarily those who practiced them, and one must not underestimate the secretive nature of these valuable skills.

gems, corundum, USA.12, Montana.12, chemistry, geology.11

Montana hosts the largest sapphire deposits in the US, but the genesis of and connection among the various secondary and primary sapphire occurrences remain cryptic. In situ SIMS measurements of oxygen isotopes in sapphires and zircon inclusions in sapphires provide an opportunity to study the isotope and trace element geochemistry in order to understand sapphire-forming protoliths (i.e. crustal setting and alteration). Sapphire from Montana was transported as xenocrysts in carrier (host) magmas that resorbed sapphire exteriors during transport. The timing and nature of sapphire genesis is elucidated by SIMS measurements of trace elements and U–Pb from discrete zones in zircon inclusions with rims that are interpreted to be syngenetic with host sapphire. Montana sapphires exhibit a large range of δ¹⁸O values, from −3‰ to +12‰ VSMOW. However, all but two anomalous crystals fall in the range of 0‰ to 8‰. There is significant crystal-to-crystal variability yet averages at most deposits are consistent with high-temperature equilibration with the mantle (δ¹⁸O(Crn) = 4.4‰ to 5.7‰), with the exception of the commercial sapphire deposits at Rock Creek that average 2.7‰. Ruby analyses are limited, but typically have lower δ¹⁸O values compared to sapphires from the same detrital localities. Homogeneity within individual crystals (avg. 2σ = ±0.2‰) indicates the absence of isotopically distinct fluid or melt during crystallization. But intercrystalline δ¹⁸O ranges by up to 7‰ at a single locality, suggesting sapphire variability at a deposit reflects heterogeneity in the original protolith. Oxygen isotope fractionations between zircon rims and surrounding sapphire suggest comagmatic zircon inclusions and corundum equilibrated at high temperature. No correlation is seen for the degree of radiation damage and alteration of δ¹⁸O(Zrc) when zircon inclusions are surrounded and armored by sapphire. U–Pb ages and trace elements were measured in a small subset of syngenetic zircon inclusions in Dry Cottonwood Creek sapphires, revealing a Proterozoic (1778 ± 9 Ma) age for the protolith of sapphires at this locality and a likely polygenetic history. Previous work has suggested formation of these sapphires through partial melting of anorthosites and several anorthosites occur locally and match the age of zircon inclusion cores — the Boehls Butte anorthosite (∼180 km NW of Rock Creek) and the Bitterroot anorthosite (∼55 km W of Rock Creek) could correlate with Al-rich protoliths at depth. Proterozoic U–Pb ages of zircon from the Boehls Butte anorthosite (1787 ± 2 Ma) match well with the age of zircon inclusion cores in Dry Cottonwood Creek sapphires and suggest genesis in these or similar protoliths. Zircon rims with Tera-Wasserburg lower intercept ages of 110 ± 9 Ma are consistent with previous observations of a xenocrystic relationship to the ∼50 Ma Eocene volcanic rocks. Corundum that formed over 50 Ma prior to being scavenged by Eocene magmas likely originated by the anatexis of Precambrian anorthosites and possibly other aluminum-rich rocks at depth.

gems, gemology, basic gemology, advanced gemology, gem testing, spectroscopy, chemistry, chemical analysis, corundum, X-ray diffraction

The use of photoluminescence imaging for gemstone characterization is reviewed, considering both fluorescence and its time-delayed counterpart, phosphorescence. Luminescence results from the excitation of atomic impurities and defects by an external source. Fluorescence can be excited by ultraviolet, visible, or infrared light, or even X-rays. Fluorescence to long-wave UV light is a characteristic included in diamond grading reports issued by major gemological laboratories. This article provides a comprehensive overview of the principles, mechanisms, and characteristics of luminescence that create the impressive and memorable glow of gemstones. Although diamond is the focus, a variety of colored stones and pearls are reviewed as well. This article is intended to foster a deeper appreciation of the complexity necessary to understand these natural wonders.