Special Issue "Gemstone Analysis by Spectroscopy and Microscopy"

A special issue of Minerals (ISSN 2075-163X). This special issue belongs to the section "Crystallography and Physical Chemistry of Minerals & Nanominerals".

Deadline for manuscript submissions: closed (2 July 2021).

Special Issue Editor

Dr. Thomas Hainschwang
E-Mail Website1 Website2
Guest Editor
GGTL Laboratories, Balzers, Liechtenstein
Interests: fancy colour diamonds (spectroscopy and imaging; treatments); gemstone spectroscopy; gem testing instrument developments; gem treatments

Special Issue Information

Dear colleagues,

The analysis of gemstones used to be a task mostly performed on basic optical instruments such as refractometers by gemmologists. This has changed significantly over the past several decades, and gem testing has become a highly sophisticated field of science that combines mineralogy, geology, chemistry, and physics. Gem materials are typically split into diamonds (colourless and coloured), coloured stones (all other gemstones) and organic materials such as pearls, amber and coral. Gemstone analysis involves a range of tasks including identification, treatment detection, country of origin detection, the determination of colour-causing mechanisms, the analysis of impurities and defects, plus the analysis of the growth mechanisms responsible for the specific physical characteristics of gem materials.

This Special Issue focusses on the spectroscopic and microscopic analysis of gemstones, including all types of spectroscopic and microscopic techniques used in the characterisation of gem materials. Papers are welcome that cover new analytical results obtained from testing gem materials by spectroscopy and/or microscopy, including new or little-exploited methods. Such results may include—amongst others—the characterization of specific gem materials, defect characterization of gemstones, the characterization and identification of gem treatments, and the application of new or little-exploited spectroscopic or microscopic analytical techniques. Submitted papers may cover any type of gem material, including natural, synthetic and artificial gemstones, untreated and treated.

Dr. Thomas Hainschwang
Guest Editor

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Minerals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • defect characterization
  • gem material characterization
  • treatment characterization
  • colour origin
  • spectroscopy
  • microscopy

Published Papers (10 papers)

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Research

Article
Characterization of Coloured Gemstones by X-ray Micro Computed Tomography
Minerals 2021, 11(2), 178; https://doi.org/10.3390/min11020178 - 08 Feb 2021
Viewed by 923
Abstract
The monetary value of gemstones is based on five variables: rarity, cut, weight, color and clarity. The latter refers to internal impurities and defects. Fashion may also dictate demand and price. To enhance some of these features and value, gemstones are treated. Disclosure [...] Read more.
The monetary value of gemstones is based on five variables: rarity, cut, weight, color and clarity. The latter refers to internal impurities and defects. Fashion may also dictate demand and price. To enhance some of these features and value, gemstones are treated. Disclosure or nondisclosure thereof has been controversial and affected consumer confidence. Most of these treatments are difficult to detect with the naked eye and accurately quantify with traditional optical and analytical methods. X-ray micro computed tomography (micro-CT or μCT) is proposed as a relatively low cost, physically non-destructive and complementary method to detect and quantify clarity enhancement and also to provide a unique 3D fingerprint of each gemstone. A collection of 14 cut colored gemstones was selected. Micro-CT scans allowed fracture detection, their distribution and calculation of filler volume as well as 3D mapping of inclusions, surface and internal imperfections and artificially induced modifications. As a result the method allows the construction of a digital twin. X-ray exposure could however induce unwanted color changes. This effect was minimized or eliminated by optimizing dosage and exposure time. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
Raman and Photoluminescence Mapping of Gem Materials
Minerals 2021, 11(2), 177; https://doi.org/10.3390/min11020177 - 08 Feb 2021
Cited by 4 | Viewed by 1254
Abstract
Raman and photoluminescence (PL) mapping is a non-destructive method which allows gemologists and scientists to evaluate the spatial distributions of defects within a gem; it can also provide a method to quickly distinguish different species within a composite gem. This article provides a [...] Read more.
Raman and photoluminescence (PL) mapping is a non-destructive method which allows gemologists and scientists to evaluate the spatial distributions of defects within a gem; it can also provide a method to quickly distinguish different species within a composite gem. This article provides a summary of this relatively new technology and its instrumentation. Additionally, we provide a compilation of new data for various applications on several gemstones. Spatial differences within diamonds can be explored using PL mapping, such as radiation stains observed on the rough surface of natural green diamonds. Raman mapping has proven useful in distinguishing between omphacite and jadeite within a composite of these two minerals, identifying various tourmaline species within a heterogeneous mixture, and determining the calcium carbonate polymorphs in pearls. Additionally, it has potential to be useful for country-of-origin determination in blue sapphires and micro-inclusion analysis. As new avenues of research are explored, more applications for gem materials will inevitably be discovered. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
Silicon-Oxygen Region Infrared and Raman Analysis of Opals: The Effect of Sample Preparation and Measurement Type
Minerals 2021, 11(2), 173; https://doi.org/10.3390/min11020173 - 07 Feb 2021
Cited by 1 | Viewed by 920
Abstract
An extensive infrared (IR) spectroscopy study using transmission, specular and diffuse reflectance, and attenuated total reflection (ATR) was undertaken to characterise opal-AG, opal-AN (hyalite), opal-CT and opal-C, focussing on the Si-O fingerprint region (200–1600 cm−1). We show that IR spectroscopy is [...] Read more.
An extensive infrared (IR) spectroscopy study using transmission, specular and diffuse reflectance, and attenuated total reflection (ATR) was undertaken to characterise opal-AG, opal-AN (hyalite), opal-CT and opal-C, focussing on the Si-O fingerprint region (200–1600 cm−1). We show that IR spectroscopy is a viable alternative to X-ray powder diffraction (XRD) as a primary means of classification of opals even when minor levels of impurities are present. Variable angle specular reflectance spectroscopy shows that the three major IR bands of opal are split into transverse optical (TO) and longitudinal optical (LO) components. Previously observed variability in powder ATR is probably linked to the very high refractive index of opals at infrared wavelengths, rather than heterogeneity or particle size effects. An alternative use of ATR using unpowdered samples provides a potential means of non-destructive delineation of play of colour opals into opal-AG or opal-CT gems. We find that there are no special structural features in the infrared spectrum that differentiate opal from silica glasses. Evidence is presented that suggests silanol environments may be responsible for the structural differences between opal-AG, opal-AN and other forms of opaline silica. Complementary studies with Raman spectroscopy, XRD and scanning electron microscopy (SEM) provide evidence of structural trends within the opal-CT type. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
New Insights for Gem-Quality Mn-Bearing Diopside-Omphacite, Violane Variety, from Saint Marcel (Val D’Aosta, Italy): Its Trace Elements and Spectroscopic Characterization
Minerals 2021, 11(2), 171; https://doi.org/10.3390/min11020171 - 07 Feb 2021
Cited by 1 | Viewed by 576
Abstract
This study proposes new data on the rare pyroxene, variety violane, sampled from its type locality, Praborna manganese deposit, near Saint Marcel (Val d’Aosta, Italy). Violane is very appreciated as a gemstone for its different hues of violet-blue color and is characterized by [...] Read more.
This study proposes new data on the rare pyroxene, variety violane, sampled from its type locality, Praborna manganese deposit, near Saint Marcel (Val d’Aosta, Italy). Violane is very appreciated as a gemstone for its different hues of violet-blue color and is characterized by its diopsidic or omphacitic composition. To assess the possible causes of color, electron-probe microanalysis (EMPA) and laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) were used to establish the chemical composition. Raman and Fourier transform infrared (FTIR) spectroscopy, ideal for the non-destructive analysis, identified the different present phases directly on gemstone. Raman and FTIR spectra highlighted the presence, in the same sample, of diopside and omphacite showing almost inappreciable violet hue difference. The two minerals were easily differentiated by microprobe analyses and showed a compositional heterogeneity not linked to the different colors. The best way to detect the color-causing elements in microcrystalline violanes resulted in the analysis of trace elements and their quantification. An enrichment of Ti and Li characterizes darker violet omphacite and that of V and rare-earth elements (REE) the lilac-lavender or light violet-blue diopside. In general, our results led us to say that the color changes, previously proposed as due to Mn both in divalent and trivalent oxidation state, may be controlled by trace elements or by concentration of minor elements, such as Fe, and their oxidation state. REE patterns showed a negative anomaly of Ce that could be ascribed to the variation of the oxygen chemical potential occurring in the ore. The new data, combined with previous results, may provide new constrains on the processes that generated the Mn-rich deposit of Praborna. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
Optical and Luminescence Spectroscopy of Varicolored Gem Spinel from Mogok, Myanmar and Lục Yên, Vietnam
Minerals 2021, 11(2), 169; https://doi.org/10.3390/min11020169 - 07 Feb 2021
Cited by 1 | Viewed by 665
Abstract
We studied 12 crystal fragments of natural spinel from Mogok, Myanmar and Lục Yên, Vietnam. All samples were crystal fragments of various shapes and sizes and several of them had gemological quality. Studied samples are enriched in Cr, V, Fe2+, Fe [...] Read more.
We studied 12 crystal fragments of natural spinel from Mogok, Myanmar and Lục Yên, Vietnam. All samples were crystal fragments of various shapes and sizes and several of them had gemological quality. Studied samples are enriched in Cr, V, Fe2+, Fe3+, Zn, which are responsible for its resulting color. They could be divided into groups of V-Cr spinels with Cr 0.001–0.006 apfu, V 0.001–0.004 apfu, and Fe spinels containing increased Fe2+ (0.001–0.017 apfu) and Fe3+ (0.004–0.012 apfu). Some samples show luminescence bands at 677, 685, 697, 710, and 718 nm assigned to Cr3+. The optical absorption spectra of spinels were divided into two groups of V-Cr and Fe spinels based on the dominant element acting on optical spectra. The optical spectrum of V-Cr spinels can be divided into two zones (1) 420–550 nm (V3+ and Cr3+ absorption); (2) 640–1000 nm (Fe2+-Fe3+ charge transfer). The optical absorption spectra of Fe spinels can also be divided into two zones (1) 410–650 nm (Fe2+-Fe3+ charge transfer) and (2) 770–1000 nm (Fe2+). This variation in chromophores results in the differences in color: V-Cr spinels are pink to red, Fe spinels are in shades of blue as well as yellow and pink. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
Gemology, Mineralogy, and Spectroscopy of an Attractive Tremolitized Diopside Anorthosite Gem Material from the Philippines: A New Type of Material with Similarities to Dushan Jade
Minerals 2021, 11(2), 152; https://doi.org/10.3390/min11020152 - 31 Jan 2021
Viewed by 825
Abstract
In recent years, a new type of material called Philippines “Dushan jade” has appeared in the gemstone market in China. This new type of material, very similar in appearance and physical properties to Dushan jade, an important ancient jade with a long history [...] Read more.
In recent years, a new type of material called Philippines “Dushan jade” has appeared in the gemstone market in China. This new type of material, very similar in appearance and physical properties to Dushan jade, an important ancient jade with a long history in China, is causing confusion in the market and poses identification difficulties. Microscopy, electron probe microanalysis, Fourier transform infrared (FTIR) spectroscopy, Raman microprobe spectroscopy, and ultraviolet-visible (UV-Vis) spectroscopy were used to study the gemology, mineralogy, and spectroscopy of rock samples from the Philippines in order to differentiate them from Dushan jade. The studies showed that Philippines rock is composed mainly of anorthite and minor amounts of diopside, tremolite, uvarovite, titanite, chromite, zoisite, prehnite, thomsonite-Ca, and chlorite, among which uvarovite, diopside, and tremolite are the main color causing minerals. The origin of the color is related to the electronic transitions involving Cr3+, Fe2+, Fe3+, and charge transfer between the ions. The paragenetic mineral formation sequence of Philippines rock can be divided into three stages: (1) the magmatic stage: anorthite phenocryst, diopside, chromite, and titanite are formed first in the magma; (2) the metamorphic stage: anorthite phenocryst undergo fracture and recrystallization; the early fluid intrusion transforms diopside into tremolite forming uvarovite-grossular-andradite solid-solution around the anorthite and chromite; and (3) the late hydrothermal stage: the late hydrothermal solution fills in fractures with prehnite, thomsonite-Ca, and zoisite being formed. From the comparison studies, it was established that Philippines rock and Dushan jade are two completely different type of material. Philippines rock should be called “tremolitized diopside anorthosite”. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
The REE-Induced Absorption and Luminescence in Yellow Gem-Quality Durango-Type Hydroxylapatite from Muránska Dlhá Lúka, Slovakia
Minerals 2020, 10(11), 1001; https://doi.org/10.3390/min10111001 - 11 Nov 2020
Viewed by 764
Abstract
In talc-magnesite veins in serpentinite near Muránska Dlhá Lúka (MDL), Slovakia, yellow euhedral to subhedral crystals apatite of a gem quality occur. It has a composition of hydroxylapatite with F varying between 0.29 and 0.34 apfu, Cl in range of 0.02–0.05 [...] Read more.
In talc-magnesite veins in serpentinite near Muránska Dlhá Lúka (MDL), Slovakia, yellow euhedral to subhedral crystals apatite of a gem quality occur. It has a composition of hydroxylapatite with F varying between 0.29 and 0.34 apfu, Cl in range of 0.02–0.05 apfu and calculated OH content between 0.62–0.68 apfu. Moreover, [CO3]2− molecules were identified by FTIR and Raman spectroscopy. MDL apatite contains only up to 0.003 apfu As5+ and Si4+ substituting for P5+; Ca is substituted by small amount of Na, Fe2+, Mn2+ (all up to 0.006 apfu), and Rare Earth Elements (REE—in total up to 0.017 apfu). Compared to trace-element composition of similar apatites from Durango, Mexico, the REE content in MDL apatite is around ten times lower with Nd > Ce >> La, its chondrite-normalized REE pattern has almost a horizontal slope and larger negative Eu anomaly. The MDL apatite is richer in Mn, Pb and Li, but poorer in As, V, Th and U. The concentrations of Sr and Y are similar. In the optical absorption spectra, the most prominent bands are at 585–590 nm (Nd3+) and between 600 and 800 nm (Mn2+, Ce3+-SiO3− photochromic center and Nd3+). The photoluminescence spectrum of MDL apatite shows bands between 550 and 620 nm (Dy3+, Sm3+, Pr3+ and also Mn2+) which likely enhance its yellow color. MDL hydroxylapatite likely formed from fluids derived from granitic rocks as evidenced by the chondrite-normalized REE patterns, Li, Mn and Y concentrations. The Sr content reflects the host-rock serpentinite composition. Fluids for its crystallization were likely derived from Muráň complex orthogneisses by the Alpine deformation and recrystallization in greenschist to lower amphibolite facies. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
A Defect Study and Classification of Brown Diamonds with Non-Deformation-Related Color
Minerals 2020, 10(10), 914; https://doi.org/10.3390/min10100914 - 15 Oct 2020
Cited by 9 | Viewed by 1203
Abstract
While the first part of this study took a detailed look at the properties, defects and classification of brown diamonds with deformation-related (DR) brown color and compared them to pink to purple to red diamonds, this second part covers diamonds with non-deformation-related (referred [...] Read more.
While the first part of this study took a detailed look at the properties, defects and classification of brown diamonds with deformation-related (DR) brown color and compared them to pink to purple to red diamonds, this second part covers diamonds with non-deformation-related (referred to as NDR in this study) brown color, including diamonds with treatment-induced brown color and synthetic brown diamonds. It was found that the natural NDR brown diamonds include CO2 and Pseudo CO2 diamonds as well as certain hydrogen-rich diamonds. Based on these, the new classification of NDR brown diamonds has been elaborated, resulting in 5 different classes. The detailed defect study performed has shown and confirmed the complexity of the CO2 and Pseudo CO2 diamonds; the probable link between structurally bound oxygen and some of the spectroscopic features such as the 480 nm absorption band is apparent in these diamonds. One of the most interesting findings was made through the low temperature NIR spectroscopy of some usually hydrogen-rich diamonds, which has defined a defect of great interest, the 1330 nm center; we suggest that this defect, together with the many lines in the 970 to 1000 nm range—referred to as the 990 nm series in this study—are responsible for the complex UV-Vis-NIR spectra seen of these diamonds. The results indicate that both features are nickel-nitrogen-related defects, the 1330 nm defect without involvement of hydrogen and the 990 nm series likely with hydrogen involved. Another surprising result was that during various treatment experiments performed we created dark orangish brown color in originally pale yellow “cape” diamonds by HPHT treatment at 2500 °C. It is suggested that the creation of this brown hue is related to the destruction or transformation of the N3 center at such extreme conditions. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
A Defect Study and Classification of Brown Diamonds with Deformation-Related Color
Minerals 2020, 10(10), 903; https://doi.org/10.3390/min10100903 - 12 Oct 2020
Cited by 7 | Viewed by 1599
Abstract
For this study, the properties of a large sample of various types of brown diamonds with a deformation-related (referred to as “DR” in this work) color were studied to properly characterize and classify such diamonds, and to compare them to pink to purple [...] Read more.
For this study, the properties of a large sample of various types of brown diamonds with a deformation-related (referred to as “DR” in this work) color were studied to properly characterize and classify such diamonds, and to compare them to pink to purple to red diamonds. The acquisition of low temperature NIR spectra for a large range of brown diamonds and photoexcitation studies combined with various treatment experiments have opened new windows into certain defect characteristics of brown diamonds, such as the amber centers and naturally occurring H1b and H1c centers. It was determined that the amber centers (referred to as “AC” in this work) exhibit rather variable behaviors to annealing and photoexcitation; the annealing temperature of these defects were determined to range from 1150 to >1850 °C and it was found that the 4063 cm−1 AC was the precursor defect of many other ACs. It is suggested that the amber centers in diamonds that contain at least some C centers are essentially identical to the ones seen in diamonds without C centers, but that they likely have a negative charge. The study of the naturally occurring H1b and H1c link them to the amber centers, specifically to the one at 4063 cm−1. Annealing experiments have shown that the H1b and H1c defects and the 4063 cm−1 AC were in line with each other. The obvious links between these defects points towards our suggestion that the H1b and H1c defects are standalone defects that consist of multiple vacancies and nitrogen and that they are—in the case of brown diamonds—a side product of the AC formation. A new classification of DR brown diamonds was elaborated that separates the diamonds in six different classes, depending on type and AC. This classification had been completed recently with the classification of brown diamonds with a non-deformation-related color (referred to as “NDR”), giving a total of 11 classes of brown diamonds. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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Article
Real-Time Detection of Long Lived Near Infrared luminescence from Colourless Cubic Zirconia by Time-Gated Imaging
Minerals 2020, 10(10), 891; https://doi.org/10.3390/min10100891 - 08 Oct 2020
Cited by 1 | Viewed by 1163
Abstract
Here, we report a long-lived ms time scale decay luminescing in the near infrared >800 nm present in productions of ‘white’ colourless, facetted yttrium stabilized cubic zirconia and observed using time-gated imaging. The spectrum of the strong luminescing feature also has characteristics of [...] Read more.
Here, we report a long-lived ms time scale decay luminescing in the near infrared >800 nm present in productions of ‘white’ colourless, facetted yttrium stabilized cubic zirconia and observed using time-gated imaging. The spectrum of the strong luminescing feature also has characteristics of Neodymium (Nd3+) and has a multiexponential decay behaviour. Real-time detection of cubic zirconia mounted in diamond jewellery containing very small stones (≤0.01 ct) is made possible, where observation by loupe is more challenging or other conventional techniques impractical and or slow to implement. The near infrared observed can be excited using a low-cost and eye/skin safe-visible green LED light source and the time-gated imaging of the luminescence using a machine vision monochrome camera. The use of near infrared, time-gated detection in combination with other verification instruments increases the robustness of screening diamond parcels. Therefore, it is recommended that any stone exhibiting strong delayed luminescence in the near infrared be treated with caution, as this is not a typical feature found in this precious gemstone. In this case, the instrument developed was expanded to incorporate a white LED illumination ring as a viewfinder, in order to aid the inspection of loose and mounted configurations. Full article
(This article belongs to the Special Issue Gemstone Analysis by Spectroscopy and Microscopy)
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