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Crystals
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Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology
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Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Mineralogical Crystallography and Biomineralization".

Deadline for manuscript submissions: closed (31 August 2024) | Viewed by 12021

Special Issue Editors

Prof. Dr. Xiaoyan Yu

E-Mail Website
Guest Editor
School of Gemology, China University of Geosciences, Beijing 100083, China
Interests: gemmology; mineralogy; spectroscopy; geochemistry; geographic origin; gem identification; in-situ micro-analysis
Dr. Fei Liu

E-Mail Website
Guest Editor
Institute of Geology, Chinese Academy of Geological Sciences, Xicheng District, Beijing, China
Interests: diamond-hosted in ophiolite and kimberlite; gem geology
Special Issues, Collections and Topics in MDPI journals
Dr. Cun Zhang

E-Mail Website
Guest Editor
School of Materials Science and Engineering, Qilu University of Technology, Jinan 250353, China
Interests: gemology; mineralogy; metamorphism; fluid inclusions; isotope geochemistry; carbon
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Gemstones, valued for their beauty, durability, and rarity, have long been highly prized resources. Natural gem crystals form under specific physical and chemical conditions; attempts to replicate these conditions can create synthetic gemstones. However, numerous aspects of the formation process and mechanism for both natural and synthetic gemstones remain enigmatic and warrant further investigation. Gemstones also embody the significance of mineralogy and geology, shedding light on the Earth's evolution through their crystal structures, defects, chemistry, and inclusions. With the ever-growing interest around their origin, formation, and identification, the pursuit of knowledge in this field has never been more crucial.

This Special Issue is focused on relevant topics, including but not limited to: 1) unravelling the natural gemstone coloration, treatment, identification, and geographic origin; 2) probing synthetic gemstone growth conditions, methods for optimizing crystals, and identification techniques; 3) tracing the metamorphic pathways and the geological events that lead to natural gem formation; 4) archeogemology involving historical significance and ancient techniques, providing a temporal perspective on gemstone usage; and 5) emerging discoveries from globally renowned gem localities and excavation sites.

Prof. Dr. Xiaoyan Yu
Dr. Fei Liu
Dr. Cun Zhang
Guest Editors

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. Crystals 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 2100 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

  • gemology
  • gemstone characterization
  • geographic origin
  • gem identification
  • in-site micro-analysis
  • spectroscopy
  • synthetics
  • treatment
  • geochronology

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Published Papers (7 papers)

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Research

11 pages, 5167 KiB  
Article
Unveiling the Bluish Green Chalcedony Aquaprase™—The Study of Its Microstructure and Mineralogy
by Sara Monico, Ilaria Adamo, Valeria Diella, Yianni Melas, Loredana Prosperi and Nicoletta Marinoni
Crystals 2024, 14(11), 1003; https://doi.org/10.3390/cryst14111003 - 19 Nov 2024
Viewed by 249
Abstract
A bluish green chalcedony (a micro to crypto polycrystalline form of silica) from Africa has been marketed with the trademark AQUAPRASETM. A multimethodological approach, combining gemological analyses, thin section examination, scanning electron microscopy, X-ray powder diffraction, Raman spectroscopy, and trace elements [...] Read more.
A bluish green chalcedony (a micro to crypto polycrystalline form of silica) from Africa has been marketed with the trademark AQUAPRASETM. A multimethodological approach, combining gemological analyses, thin section examination, scanning electron microscopy, X-ray powder diffraction, Raman spectroscopy, and trace elements chemical analyses by LA–ICP–MS, was carried out to characterize this material from a gemological and mineralogical point of view. The chalcedony samples consist of a mixture of quartz and moganite, as shown by the X-ray powder diffraction analysis and Raman spectroscopy. “Aquaprase” showed a strong microstructural zoning in terms of grain size, from macrocrystalline to micro and crypto, and morphology. Trace element variations correlated well with the different colored areas of the samples. In particular, the main chromophore ion present in the bluish green areas of the “aquaprase” chalcedony was chromium, followed by iron and nickel, so this chalcedony could be included in the group of chromium-bearing chalcedony. Rayleigh light scattering contributed to the blue hue of the gems. Full article
(This article belongs to the Special Issue Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology)
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11 pages, 3571 KiB  
Article
Near-Infrared Spectroscopic Study of Secondary Minerals in the Oxidation Zones of Copper-Bearing Deposits
by Shaokun Wu, Mingyue He, Mei Yang, Bijie Peng, Yujia Shi and Kaiyue Sun
Crystals 2024, 14(9), 791; https://doi.org/10.3390/cryst14090791 - 7 Sep 2024
Viewed by 597
Abstract
This study measured the infrared spectra of secondary minerals in the oxidation zones of three types of copper ores: dioptase, malachite, and azurite, and assigned the peak positions of OH stretching vibrations and the origins of OH combination vibrations. Dioptase contains three types [...] Read more.
This study measured the infrared spectra of secondary minerals in the oxidation zones of three types of copper ores: dioptase, malachite, and azurite, and assigned the peak positions of OH stretching vibrations and the origins of OH combination vibrations. Dioptase contains three types of water molecules with different orientations within its ring channels, which exhibit six kinds of OH stretching vibrations in the 3000–3600 cm−1 range; the bond length range is 2.652 to 2.887 Å. Among them, the 3443 cm−1 band shows strong near-infrared activity and combines with Si–O vibrations or OH bending vibrations in the structure, resulting in five combination vibration peaks in the 4000–5000 cm−1 range. Malachite contains two inequivalent hydroxyls in its structure, leading to two OH stretching vibrations in the high-frequency region located at 3314 and 3402 cm−1, respectively. Azurite contains only one type of hydroxyl, and thus only one characteristic OH stretching vibration is present at 3424 cm−1. The OH stretching vibrations of malachite and azurite mainly combine with [CO3]2− vibrations or OH bending vibrations, leading to six and five combination peaks in the OH combination vibration region, respectively. By analyzing the combination of peak positions at 4341 cm−1 in the near-infrared spectrum, the merged OH bending vibration at 921 cm−1 in azurite was discovered. Spectroscopic research on secondary minerals can better provide a basis for ore exploration and geological remote sensing. Full article
(This article belongs to the Special Issue Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology)
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11 pages, 3923 KiB  
Article
Identification of Some Gem-Quality Red and Green Feldspars
by Zhongyi Shang, Zhiqing Zhang and Qingchao Zhou
Crystals 2024, 14(5), 409; https://doi.org/10.3390/cryst14050409 - 27 Apr 2024
Viewed by 1109
Abstract
Sunstone is a member of the feldspar group. Natural sunstones from Oregon exhibit unique optical effects and hold significant market value. However, since 2008, there has been a persistent issue of diffused red feldspars masquerading as natural sunstones in the market, severely undermining [...] Read more.
Sunstone is a member of the feldspar group. Natural sunstones from Oregon exhibit unique optical effects and hold significant market value. However, since 2008, there has been a persistent issue of diffused red feldspars masquerading as natural sunstones in the market, severely undermining consumer confidence in purchasing natural sunstones. Fluorescence characteristics under 305–335 nm ultraviolet excitation are considered an effective method for distinguishing copper-diffused red feldspars from natural sunstones. In this paper, through detailed analysis and testing of ten market-acquired red and green feldspar samples, including UV-vis spectra, microscopic characteristics, fluorescence spectra, and chemical compositions, we validate the efficacy of fluorescence characteristics in identifying copper-diffused feldspars. The results verify the widespread prevalence of copper diffusion treatment in market-acquired red and green feldspars, shedding light on their treatment history and providing valuable insights for jewelry consumers. This research not only enhances our understanding of sunstone treatments but also strengthens the reliability and applicability of fluorescence spectroscopy in gemstone identification, offering promising prospects for its broader adoption in the jewelry market. Full article
(This article belongs to the Special Issue Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology)
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23 pages, 15900 KiB  
Article
Ruby from Longido, Tanzania: Mining, Color, Inclusion, and Chemical Features
by Yujie Gao, Mingyue He, Andrew Christopher Lucas, Xueying Sun, Dan Zhou, Tiantian Huang, Kai Li, Darwin Fortaleché, Moqing Lin, Yuan Zhu and Xiaoting Jin
Crystals 2024, 14(4), 383; https://doi.org/10.3390/cryst14040383 - 19 Apr 2024
Viewed by 1995
Abstract
This article reports on the recent mining and production status of ruby in Longido, Tanzania. Faceted-grade rubies and their matrix from Longido Area, Tanzania, were investigated by standard gemological testing, including FTIR, UV-VIS, Raman spectra, and LA-ICP-MS. Microscopic observations revealed dense needle-like and [...] Read more.
This article reports on the recent mining and production status of ruby in Longido, Tanzania. Faceted-grade rubies and their matrix from Longido Area, Tanzania, were investigated by standard gemological testing, including FTIR, UV-VIS, Raman spectra, and LA-ICP-MS. Microscopic observations revealed dense needle-like and triangular inclusions, distinct growth lines, and color banding as typical inclusions. In agreement with the Raman results, the transmission FTIR spectrum confirmed the presence of aluminum hydroxide. The Raman spectra identified associated minerals and inclusions, including zoisite, parasites, feldspar within the matrix, rutile, and diaspore in the ruby host. The chemistry analysis revealed a high amount of Cr and relatively low iron as a good indicator of geographic origin. Full article
(This article belongs to the Special Issue Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology)
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15 pages, 5666 KiB  
Article
Inclusions and Spectral Characterization of Demantoid from Baluchistan, Pakistan
by Jian-Yi Zhang, Geng Li, Yu Tian and Fabian Schmitz
Crystals 2024, 14(1), 84; https://doi.org/10.3390/cryst14010084 - 16 Jan 2024
Cited by 7 | Viewed by 1446
Abstract
Demantoid is the green variety of andradite [Ca3Fe2(SiO4)3], an exceptionally rare and precious gemstone worldwide. In recent years, a small amount of gem-quality demantoid has been found in Pakistan. This research focuses on nine demantoids [...] Read more.
Demantoid is the green variety of andradite [Ca3Fe2(SiO4)3], an exceptionally rare and precious gemstone worldwide. In recent years, a small amount of gem-quality demantoid has been found in Pakistan. This research focuses on nine demantoids sourced from Muslim Bagh, Baluchistan, Pakistan, presenting a comprehensive analysis of the spectral characteristics and inclusions of Pakistani demantoid using classical gemological methods, energy dispersive X-ray fluorescence (EDXRF) chemical analyses, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and ultraviolet and visible (UV-vis) spectroscopy. The results show that the content of Cr and V in most samples is lower than the detection line of EDXRF, with only one sample containing a Cr2O3 content of 0.032%. The extremely low Cr content sets Pakistani demantoid apart from demantoid of the serpentinite type found in other regions. Notably, the UV-vis spectrum reveals characteristic absorption at 443 nm due to Fe3+, while a further contribution from Cr3+ would be highly likely, and weak absorption at 550 nm caused by Fe3+. This suggests that iron (Fe) is the primary chromogenic element of Pakistani demantoid, but the role of Cr3+ cannot be ignored. The FTIR spectrum of Pakistani demantoid displays the absorption peaks associated with [SiO4]4− groups at 937 cm−1, 848 cm−1, and 817 cm−1, while the absorption peaks resulting from trivalent cations appear at 481 cm−1 and 442 cm−1, which are the characteristic FTIR spectra of demantoid. Raman spectroscopy further reveals absorption peaks are displayed near 994 cm−1, 843 cm−1, 818 cm−1, associated with (Si–O)Str vibrations (Si–O stretching vibration), and absorption peaks are displayed near 350 cm−1 and 310 cm−1, related to the rotation of SiO4–R(SiO4)4−, and the peaks near 514 cm−1 and 494 cm−1 are related to (Si–O)bend vibrations (Si–O bending vibration). Additionally, related absorption peaks near 168 cm−1 are attributed to the translation of SiO4–T(SiO4)4−, and absorption peaks near 234 cm−1 are associated with the translation of X2+–T(X2+) (X2+ represents divalent ions). The common dark opaque inclusions found in Pakistani demantoid consist of a combination of magnetite and hematite. Additionally, some samples of Pakistani demantoid display inclusions of calcite. This unique combination of inclusions differentiates Pakistani demantoid from demantoids sourced from other regions. It signifies that Pakistani demantoid has a distinctive geological origin resulting from the interplay of serpentinization and skarnization processes. This geological formation distinguishes it from demantoids solely hosted in serpentinite or skarn environments in other origins. The identification of these characteristics holds significant importance for accurately determining the origin of Pakistani demantoid. Full article
(This article belongs to the Special Issue Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology)
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16 pages, 17859 KiB  
Article
A New Type of White Nephrite from Limestone Replacement along the Kunlun–Altyn Tagh Mountains: A Case from the Mida Deposit, Qiemo County, Xinjiang, China
by Tianlong Jiang, Guanghai Shi, Danning Ye, Xiaochong Zhang, Linjing Zhang and Hongwei Han
Crystals 2023, 13(12), 1677; https://doi.org/10.3390/cryst13121677 - 12 Dec 2023
Cited by 2 | Viewed by 1755
Abstract
The recently discovered Mida nephrite deposit, located in the East Kunlun Mountains, Qiemo County, Xinjiang, Northwest China, contains new types of white and greenish white nephrite formed by limestone replacement, which shows microstructures, macroscopic features and country rocks typologies that are quite different [...] Read more.
The recently discovered Mida nephrite deposit, located in the East Kunlun Mountains, Qiemo County, Xinjiang, Northwest China, contains new types of white and greenish white nephrite formed by limestone replacement, which shows microstructures, macroscopic features and country rocks typologies that are quite different from those of the other deposits along the Kunlun–Altyn Tagh Mountains. The gemological and mineralogical characteristics of Mida nephrite are presented here. These nephrites show an ivory white color and a porcelain-like appearance, with semitranslucent-to-opaque transparency and a porcelain-to-greasy luster. Petrographic study, electron probe microanalysis (EPMA) data and scanning electron microscopy (SEM) images have indicated that the nephrite is composed of tremolite, accompanied by minor quartz, calcite and diopside. Tremolite aggregates have shown different textures, like flaky, granular, fibrous–felted, bundle, radial and metasomatic relict textures. Quartz has appeared in granular or disseminated form, dispersed in the tremolite matrix. Calcite has shown a metasomatic relict texture in the white nephrite samples. Diopside has shown euhedral grains, with some distributed with a certain geometric appearance. Based on our observations, it is suggested that the quartz in the nephrite originated from Si-rich hydrothermal fluids. We propose that the substantial size difference of mineral grains, together with uncompacted grains with inter-particle pores, are the main reasons for the internal reflection and refraction under transmitted light, which allow less transmitted light to pass through the nephrite body and generate the appearance of a semitranslucent-to-opaque transparency, ivory white color and porcelain luster. Our study has unveiled that the Mida nephrite is not typical of the two known types (D-type: dolomite-related; S-type: serpentinite-related) and is overlapped by quartz grains dispersed throughout the less compact tremolite matrix. These observations would help set it apart from the majority of nephrite jades found in the Kunlun Mountains region and provide valuable insights for enhancing comprehension of the diversity of the nephrite deposits. Full article
(This article belongs to the Special Issue Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology)
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17 pages, 3724 KiB  
Article
Color-Causing Mechanisms of Guatemala Jadeite Jade: Constraints from Spectroscopy and Chemical Compositions
by Ting Li, Cun Zhang, Linsu Lv, Haitao Zhang, Yuqing Chen, Zhibin Li and Yue Liu
Crystals 2023, 13(11), 1535; https://doi.org/10.3390/cryst13111535 - 26 Oct 2023
Cited by 1 | Viewed by 3244
Abstract
The jadeite jade in Guatemala exerts remarkable commercial quality, which has attracted wide attention. Guatemalan jadeite jade displays a rich variety of colors; however, the color formation of this jadeite jade has not been systematically investigated to date. In this paper, we study [...] Read more.
The jadeite jade in Guatemala exerts remarkable commercial quality, which has attracted wide attention. Guatemalan jadeite jade displays a rich variety of colors; however, the color formation of this jadeite jade has not been systematically investigated to date. In this paper, we study different colors of jade samples to trace the compositions and color-causing mechanisms through petrography, X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy (LRS), and UV-visible absorption spectroscopy (UV-Vis), as well as electron probe microanalysis (EPMA). The results show that jadeite and omphacite are the main mineral compositions of Guatemalan jadeite jade, together with minor albite and other impurities. The color of Guatemala jadeite jade is mainly related to Cr3+, Fe2+, and Fe3+, of which a small amount of Cr3+ causes the jadeite jade to be emerald green. Moreover, 1~2% FeO contents can lead to the blue or gray color of the samples, while the Fe3+ makes the sample dark green. The green color of some Cr3+-free jadeite is caused by the electron transition between bands of Fe3+, and the green color is related to the iron content. Moreover, the chemical composition analysis shows that some metallic elements existed in Guatemalan jadeite jade, such as Ca, Ti, Al, Si, Ni, Fe, Mn, Cr, Na, Mg, and Sr, and some trace elements were lost or unevenly distributed, which may lead to the heterogeneity of the color of the samples. Our present investigation provides insights into color discrimination, quality evaluation, and identification of Guatemala jadeite jade. Full article
(This article belongs to the Special Issue Gems Decoded: Bridging Gemology, Mineralogy, Crystallography and Geology)
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