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: 31 August 2024 | Viewed by 4205

Special Issue Editors

School of Gemology, China University of Geosciences, Beijing 100083, China
Interests: gemmology; mineralogy; spectroscopy; geochemistry; geographic origin; gem identification; in-situ micro-analysis
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
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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Keywords

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

Published Papers (4 papers)

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Research

15 pages, 5666 KiB  
Article
Inclusions and Spectral Characterization of Demantoid from Baluchistan, Pakistan
Crystals 2024, 14(1), 84; https://doi.org/10.3390/cryst14010084 - 16 Jan 2024
Cited by 1 | Viewed by 569
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
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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
Crystals 2023, 13(12), 1677; https://doi.org/10.3390/cryst13121677 - 12 Dec 2023
Viewed by 718
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
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12 pages, 5986 KiB  
Article
Spectroscopy and Trace-Element Characteristics of Emeralds from Kamakanga, Zambia
Crystals 2023, 13(11), 1605; https://doi.org/10.3390/cryst13111605 - 20 Nov 2023
Viewed by 726
Abstract
Currently, Zambia is the second largest source of emeralds, after Colombia. In this study, emerald samples from the Zambian Kamakanga deposit were examined by UV-Vis-NIR, Miro-FTIR, Diamond ViewTM, and LA-ICP-MS. Representative UV-Vis-NIR spectra showed a distinct Fe3+ absorption peak, and [...] Read more.
Currently, Zambia is the second largest source of emeralds, after Colombia. In this study, emerald samples from the Zambian Kamakanga deposit were examined by UV-Vis-NIR, Miro-FTIR, Diamond ViewTM, and LA-ICP-MS. Representative UV-Vis-NIR spectra showed a distinct Fe3+ absorption peak, and the Fe-related absorption band was much stronger than that of the Cr-related absorption band. The infrared spectra showed that the absorption of type II H2O was much stronger than that of type I H2O. The results of LA-ICP-MS indicated that darker green, green, lighter green, and bluish-green emeralds had a clear separation of Cr/V (Cr/V > 15 for darker green, 10 < Cr/V < 15 for green, and Cr/V < 10 for lighter green and bluish green). In color zoning emerald, the contents of Cr, Sc, V, and Fe gradually increased with the intensity of the green color, while the opposite occurred for Cs. Cr is the main chromogenic element in Kamakanga emeralds. Additionally, Zambian Kamakanga emeralds contain high contents of total alkali metals (avg. 17,592 ppmw), Cs (avg. 1331 ppmw), Fe (avg. 8556 ppmw), Li (avg. 485 ppmw), Li + Cs (avg. 1816 ppmw), and Ga/Fe < 0.0025. Therefore, combined Fe versus Ga, Li versus Cs binary diagrams and K, Rb, and the Li + Cs ternary plot can distinguish Zambian emeralds from other important emerald origins. Full article
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17 pages, 3724 KiB  
Article
Color-Causing Mechanisms of Guatemala Jadeite Jade: Constraints from Spectroscopy and Chemical Compositions
Crystals 2023, 13(11), 1535; https://doi.org/10.3390/cryst13111535 - 26 Oct 2023
Viewed by 1210
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
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