Sapphires have been mined in Montana since being discovered at Eldorado Bar, along the Missouri River, in 1865. The most valuable gold deposit in the area was mined near the top of the French Bar complex from 1867 to 1869. On portions of the Virginia terrace and the Center terrace, the French Bar sill, which sparsely contained sapphires in a host rock-identified here as a basaltic trachyandesite, but described elsewhere as a minette- was reported by Kunz in 1891. Other than Yogo Gulch, where the sapphires are in a lamprophyre dike, the French Bar sill is Montana's only other source of gem-quality in situ sapphire. New ownership of the property in 2020 has resulted in the largest production to date, although it is still sparse. In this study, the authors examined 80 French Bar sill sapphires in matrix; 16 sapphires removed from the matrix, ranging from very small to 17.99 ct; 4 faceted stones; and 5 partly polished crystals. Trace element chemistry and inclusions were consistent with-and generally indistinguishable from-faceted Montana sapphires from the secondary deposits of the Missouri River, Rock Creek, and Dry Cottonwood Creek. This indicates that while the magmatic material (Eocene volcanism) that transported the sapphires from deep within the earth was different in certain cases, such as with French Bar sill, the genesis of the sapphires may be the same or similar. Many of the surface resorption features are similar, but one important distinction exists. Around 25% of Missouri River alluvial sapphires have encrustations of spinel, likely a result of reaction during transport by a mafic magma. Primary French Bar sill sapphires always have an encrustation of biotite (phlogopite) mica, but never spinel. The French Bar sill itself is certainly too small in extent to have supplied all the alluvial sapphires found at the Missouri River.
In 2017, the first emerald deposit in the Qinling Orogenic Belt was found in Zhen'an County, Shaanxi Province, China. This study presents a comprehensive analysis of the gemological, spectroscopic, and trace element characteristics of a select set of Zhen'an emerald samples based on standard and advanced gemological methods, including gem microscopy, ultraviolet/visible/near-infrared spectroscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, and laser ablation-inductively coupled plasma-mass spectrometry. Microscopic studies revealed that Zhen'an emerald exhibits longitudinal striations and inclined growth steps on its hexagonal prism faces and contains an abundance of two-phase (gas/liquid) fluid inclusions. A variety of mineral inclusions were identified, including phlogopite, plagioclase, scheelite, calcite, talc, and hematite. Some crystals display color zoning, with a deeper green rim containing elevated vanadium and iron. Zhen'an emerald's dominant chromophore is vanadium, and the material is characterized by a high vanadium concentration, notably low chromium, a moderate iron concentration, and an elevated ratio of gallium to cesium. These unique geochemical signatures of Zhen'an emerald, along with their internal features, could be a robust tool for discerning emerald from this deposit.
Turquoise from the Mona Lisa mine, located in Polk County in the U.S. state of Arkansas, gained renewed interest in 2018 when Avant Mining promoted the re-opening of the mine, displaying an approximately 111 kg stabilized turquoise nugget at the AGTA GemFair in Tucson, Arizona. The unique location and geologic setting of the mine led many to question whether this material was turquoise and how it could have formed. Phosphate minerals are prevalent in Arkansas, but the lack of an obvious copper source in this area was considered an obstacle to turquoise formation. In this study, samples obtained during multiple visits to the mine were analyzed to identify their chemistry and structure. X-ray diffraction, Raman spectroscopy, ultraviolet/visible/near-infrared and Fourier-transform infrared spectroscopy, and mass spectrometry results were obtained and combined with geologic observations and measurements to comprehensively characterize Mona Lisa turquoise. Copper was identified as a major elemental component of all tested samples, with copper oxide concentrations ranging from 1.71 to 6.18 wt.%. The presence of copper in major element amounts warrants the use of the term turquoise in a gemological context, although the broad range of concentrations suggests solid-solution relationships between turquoise-group minerals, highlighting the inherent heterogeneity and complex mineralogy of turquoise deposits.
In the gemological research community, diamonds with a broad absorption band centered at roughly 480 nm in the visible spectrum are referred to as "480 nm band diamonds." Despite the 480 nm absorption band being one of the few causes of yellow bodycolor, diamonds with this feature are relatively scarce and not as widely recognized as those colored by the N3 defect, single substitutional nitrogen defect, or H3 defect. Yet 480 nm band diamonds deserve more attention because they account for the vast majority of rare diamonds with unmodified orange hues, as well as all chameleon diamonds-diamonds with a reversible color-change property. However, the global abundance and distribution of 480 nm band diamonds are currently unknown. To date, only mines in Russia and Canada are known to produce these diamonds, but other sources likely exist. Understanding their gemological and spectroscopic characteristics will help the mining industry and the gem trade identify these obscure treasures. This article summarizes the features of 480 nm band diamonds for rapid identification or advanced gemological testing.
Contact-twinned crystals of natural diamond rough, known as "macle," typically exhibit a flattened triangular shape, which has been attributed to preferential growth at re-entrant corners where two crystals mutually contact. The influence of the re-entrant corner effect on the morphological alteration of macle diamond may be closely linked to the conditions of carbon supersaturation. However, no detailed investigation has been conducted regarding the possibility of preferential growth on the opposite side of the re-entrant corner, which is referred to as a "salient corner" in this study. The four external and internal growth morphologies (I, II, III, and IV) of contact-twinned macle diamonds with four different corresponding re-entrant corner shapes (i, ii, iii, and iv) were analyzed by examining the surface features of each corner and the zonal growth structure through scanning electron microscopy-cathodoluminescence images. The study confirmed that morphological changes and the flatness of macle diamonds resulted from preferential growth at both re-entrant and salient corners. To our knowledge, this is the first reporting of group III and IV macle crystals characterized by an apex covered by high-index {hhk} faces as well as the salient corner effect. Additionally, the variations in morphology appear to correlate with fluctuations in carbon supersaturation conditions in the diamonds' growth medium.
The term orange peel effect is often used to refer to the uneven appearance of the polished surface of jadeite, which arises due to the hardness anisotropy of its mineral grains arranged in different directions. The unusual orange peel effect observed on white nephrite from Russia has been preliminarily recognized in the commercial sphere, but minimal description of the underlying causes for the effect exists in the literature. In this investigation, this orange peel surface feature, which includes granular pits and bumps, pseudo-rhombic microstructure, and subparallel fissures, was explored using gemological and petrographic microscopes, an electron probe microanalyzer with backscattered electron imaging, and cathodoluminescence imaging. The observed surface feature, related to pseudomorphs, results from metasomatism where nephrite (tremolite) replaces rhombic carbonate (dolomite) mineral grains while preserving their original microstructure. Petrographic observations show that the pseudo-rhombs on these pseudomorphs are intersected by a network of veins, similar to the framework of cleavage patterns of calcite or dolomite. The pseudo-rhombs ranged from 100 to 500 mu m in width, consisting of a thin vein outline and an inside region composed of fine compacted fibers, with a relatively less compact central domain. This study demonstrates that this effect is largely dependent on the nephrite's adopted microstructure rather than its chemical composition or mineral components. Derived from these results, a formation model of this distinctive microstructure is proposed, arising from fluid flow and precipitation accompanied by volume shrinkage reactions during replacement progress. Based on the review of the microstructures of available white nephrite samples from other localities, the pattern of pseudomorphs with pseudo-rhombs and claw lineation is a unique feature of Russian nephrite.
This article reviews all of the special optical effects of gemstones, including opalescence, chatoyancy, asterism, schiller, and iridescence. The physics of light scattering, reflection, diffraction, and interference is briefly described for qualitative explanations of these optical phenomena. The most up-to-date microscopic investigations of the submicron inclusions and nanotextures in each type of phenomenal stone, along with their mechanistic interpretations, are also summarized. Although the basic principles behind these phenomena are generally understood, quantitative descriptions that directly connect the optical effects and the submicron structures are still lacking for many of these stones. In addition, the formation mechanisms of some of the textures in phenomenal stones are still debated, if not completely elusive. This article roughly sorts the optical effects by the dimensionality and complexity of their underlying submicron structures and textures. Zero-dimensional nanoparticles in diamond and corundum, though typically not considered phenomenal gemstones, produce milky or opalescent appearances by scattering light randomly; onedimensional oriented needle-like inclusions in chrysoberyl and garnet create cat's-eye and star effects; two-dimensional platelets and layers in feldspar produce schiller and iridescence by reflection and interference; and three-dimensional photonic crystals in precious opal cause brilliant play-of-color by Laue and Bragg diffraction. The often-confusing past uses of the phenomenal terminology in the literature are also clarified in this article.
A sixteenth-century pendant on display at the Museum of the National Library of France in Paris has been linkedto Catherine de' Medici, Queen of France from 1547 to 1559, and Fran & ccedil;ois Dujardin, goldsmith to King CharlesIX beginning in 1570. The pendant is adorned with two emeralds that one historian claimed were Colombianwithout documentation or scientific proof. This study aims to confirm the nature of these emeralds and determinetheir geographic origin. The microscopic, spectroscopic, and chemical data collected in this study indicate thatthese two emeralds are from Pakistan. The presence of emeralds from Pakistan in this sixteenth-century Europeanpendant provides strong evidence of the existence of trade routes between Asia and Europe during this periodof time, or even before, and suggests such emeralds might be present in other royal jewels.
Heat treatment of some ruby and sapphire can be identified by detection of the 3309 cm-1 series in their infrared spectra. The 3309 cm-1 series consists of three peaks at 3309, 3232, and 3185 cm-1, with two additional weak peaks at 3367 and 3295 cm-1. Observation of this series in a ruby or sapphire has significantly different implications than the identification of a single peak at 3309 cm-1. This series represents a specific form of hydrogen incorporation in the corundum structure that is a diagnostic indicator of heat treatment for certain types of ruby and sapphire, particularly ruby from Mozambique and pink sapphire from Madagascar. However, identification of this series can be challenging because other peaks can occur in the same region as the 3309 cm-1 series. This article presents criteria for accurate identification of the three main peaks in the 3309 cm-1 series by measuring their possible peak position ranges, allowing for separation from other possible unrelated peaks in this region. Measurements were made on heated and unheated natural and laboratory-grown blue sapphire and ruby from various origins. With increasing iron and/or chromium content, the positions of these three main peaks shifted narrowly, about 1.7 to 3.6 cm-1, from minimum positions at 3308.9, 3231.2, and 3183.7 cm-1. A positive linear relationship was observed between the positions of three main peaks in the 3309 cm-1 series and concentrations of iron for blue sapphire or the sum of iron and chromium for ruby. These relationships are useful to determine whether peaks in this region actually represent the 3309 cm-1 series or some other hydrogen-related species in the corundum structure. Additionally, peak widths in the 3309 cm-1 series also broaden with increasing iron or the sum of iron and chromium concentrations.
Red diamonds are among the rarest gems on Earth, especially Fancy red diamonds that are pure red and unmodifiedby brown, orange, or purple. At 2.33 ct, the Winston Red diamond is the fifth-largest Fancy red diamond knownto exist and the only Fancy red diamond on public exhibit. On April 1, 2025, it was unveiled in a new exhibit atthe Smithsonian National Museum of Natural History in Washington, DC. This is the first scientific and historicalstudy conducted on this noteworthy stone. Optical observation along with spectroscopic, cathodoluminescence,and photoluminescence analyses confirmed the presence of plastic deformation bands and dislocation networkpatterns that classify the Winston Red as a type IaAB (A
X-ray radiography and X-ray computed microtomography are two imaging techniques with far-reaching applications in many fields. In gemology, they play a crucial role in the identification of pearl, a unique biogenic gem material. This article offers a brief history of pearl testing by X-ray imaging, as well as basic instrumental theory, examples of different types of pearls that can be separated with X-ray imaging, and examples of its other uses in gemology and mineralogy. The aim of this paper is to provide readers with a basic understanding of these methods and the importance of these devices in a gemological setting, particularly for pearl testing.
Since its discovery at the beginning of the nineteenth century, infrared light has proven to be a versatile tool for scientists, industrialists, and consumers. The gem industry has benefited from the power of infrared spectroscopy in gemstone identification for more than 30 years. By measuring the frequencies of vibrations of bonds between atoms in the structure of a gemstone, scientists can determine the composition and atomic arrangement of many important defects that indicate whether a gem is natural, laboratory-grown, or treated. Clever adaptations of instrument hardware and sampling techniques have allowed for efficient and accurate analysis of diamonds and many of the most important colored gems such as ruby, sapphire, emerald, and jadeite. No gemologist's toolbox is complete without an understanding of the power of infrared spectroscopy for gem identification.
One of the fundamental tasks in a modern gemological laboratory is identifying the species of a gemstone. Laboratories apply many different approaches, from routine methods used for almost every gemstone to more advanced techniques on a case-by-case basis. While standard gemological testing can accurately identify the most common gemstone species, additional testing is often required for unusual or rare stones. Two advanced analytical tools for phase identification are reviewed in this article: Raman spectroscopy and X-ray diffraction. Both methods harness photon scattering to characterize the atomic-scale structures of materials. Raman spectroscopy relies on inelastic light scattering from crystal lattice or molecular vibrations. X-ray diffraction relies on constructive interference of X-rays "reflected" from regularly spaced atomic layers of a crystal lattice. In addition to identifying mineral species, these techniques can capture information about composition, degree of crystallinity, strain, and other factors that affect a material's structure. The applications of these techniques are summarized first, followed by technical details regarding the underlying physics and instrumentation.
Metrology, the science of measurement and its application, plays a critical role in the accuracy of GIA's research and laboratory services. The metrology team closely monitors the performance of instruments and devices in all global laboratory locations, assessing accuracy (within the established measurement uncertainty of each parameter), repeatability, and reproducibility. Calibration objects traceable to a national standards agency (such as the National Institute of Standards and Technology) and working references traceable to those calibration objects provide benchmarks for these performance criteria. Instruments that do not meet these criteria are removed from service until repair and recalibration have been performed to restore measurement validity.
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.
