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Raman and Photoluminescence Mapping of Gem Materials minerals Article Raman and Photoluminescence Mapping of Gem Materials Sally Eaton-Magaña *, Christopher M. Breeding, Aaron C. Palke, Artitaya Homkrajae, Ziyin Sun and Garrett McElhenny Gemological Institute of America, Carlsbad, CA 92008, USA; [email protected] (C.M.B.); [email protected] (A.C.P.); [email protected] (A.H.); [email protected] (Z.S.); [email protected] (G.M.) * Correspondence: [email protected] 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 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. Keywords: Raman mapping; photoluminescence mapping; spectroscopy; diamond; corundum; tourmaline; jadeite; pearl; gemology Citation: Eaton-Magaña, S.; Breeding, C.M.; Palke, A.C.; Homkrajae, A.; Sun, Z.; McElhenny, G. Raman and Photoluminescence Mapping of Gem Materials. Minerals 1. Introduction 2021, 11, 177. https://doi.org/ Gemological laboratories rely on non-destructive analytical techniques that are gen- 10.3390/min11020177 erally based on optical methods such as absorption spectroscopy along with Raman and photoluminescence (PL) spectroscopy. PL and Raman spectroscopy, as used for gemstones, Academic Editor: is a microscope-assisted analytical technique in which a material is illuminated with laser Thomas Hainschwang light and the resulting emission is measured with a high-resolution spectrometer. Received: 12 January 2021 PL spectroscopy has become an indispensable tool used by major gemological labora- Accepted: 1 February 2021 tories to distinguish treated and lab-grown diamonds from their natural counterparts [1,2]. Published: 8 February 2021 Within diamond, the presence, and thus the detection, of various defects differs with its growth history along with any subsequent treatment. PL spectroscopy can detect these Publisher’s Note: MDPI stays neutral features even at concentrations lower than 10 ppb [3]. Raman spectroscopy has proven with regard to jurisdictional claims in vital for a host of other gemstones as well to aid in their identification [4–7]. published maps and institutional affil- Raman and PL mapping is a logical extension of these methods in which spectra iations. can be collected quickly and automatically across an area instead of a single point (or the sample manually moved to collect multiple spectra across a sample). These mapping techniques have become possible in the past few years as the technology for the necessary instrumentation has improved. Therefore, Raman and PL mapping have become ideal Copyright: © 2021 by the authors. methods to analyze the spatial differences in these diamonds and similar diamonds that are Licensee MDPI, Basel, Switzerland. distinguished by distinct growth regions. This new instrumentation automatically collects This article is an open access article hundreds to thousands of spectra across a sample and has provided several new, exciting distributed under the terms and research opportunities and identification avenues in recent years. conditions of the Creative Commons This article provides a brief account of investigations across a broad range of research Attribution (CC BY) license (https:// and identification interests for a wide array of different gemstones. It offers an indication creativecommons.org/licenses/by/ 4.0/). of the variety of research possibilities across the breadth of the gemological world. Minerals 2021, 11, 177. https://doi.org/10.3390/min11020177 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 34 This article provides a brief account of investigations across a broad range of research and identification interests for a wide array of different gemstones. It offers an indication Minerals 2021, 11, 177 2 of 31 of the variety of research possibilities across the breadth of the gemological world. 2. Materials and Methods 2. MaterialsGenerally, and Raman/PL Methods mapping is used to show the distribution of the peak intensity associatedGenerally, with the Raman/PL concentration mapping of an is optical used to defect; show thehowever distribution, other ofdata the such peak as intensity peak positionassociated or peak with width the concentration can be plotted of instead, an optical depending defect; however,on the application. other data A such Raman/PL as peak mappingposition microscope or peak width can can collect be plotted thousands instead, of dependingspectra in a on raster the application. pattern (Figure A Raman/PL 1) and recordmapping a spectrum microscope at each can point. collect The thousands collection ofarea spectra on the in gem’s a raster surface pattern for (Figureeach individ-1) and ualrecord spectrum a spectrum can be at quite each point.small (<1 The µ collectionm2) or quite area large on the (>1000 gem’s µm surface2) if needed for each to individual accom- 2 2 modatespectrum time can constraints be quite small or a (<1 veryµm weak) or quiteemission large signal. (>1000 Theµm )spectral if needed data to accommodate(typically a usertime-specified constraints peak or selected a very from weak the emission spectrum) signal. are then The interpolated spectral data to (typicallyproduce a a map user- ofspecified a defect’s peak distribution. selected fromWith thestandard spectrum) data collection, are then interpolated a single location to produce (represented a map by of a thedefect’s green distribution.circle) is chosen With for standard data collection data collection, which agenerally single location serves (representedas a proxy for by an the entiregreen sample circle) (although is chosen fora researcher data collection can collect which from generally multiple serves spots as on a a proxy sample for to an gauge entire thesample heterogeneity (although). With a researcher mapping can spectroscopy, collect from the multiple data collection spots on is a automated sample to such gauge that the dataheterogeneity). from the entire With surface mapping may spectroscopy, be collected the and data stitched collection together is automated, yielding such a that much data greaterfrom theand entire more surface complete may image be collected of the defects and stitched within together, the sample yielding and generally a much greater offering and thousandsmore complete of collected image spectra of the defects instead within of a single the sample spectrum. and generally offering thousands of collected spectra instead of a single spectrum. FigureFigure 1. 1.DataData collection collection across across the the table table facet facet of of a around round diamond diamond with with standard standard spectroscopy (Left) (Leftis compared) is compared against against mapping mapping spectroscopy spectroscopy in which in which spectra spectra are collectedare collected across across the entirethe entire surface surface(Right ()[Right8]. ) [8]. SomeSome of of the the considerations considerations that that we we encountered encountered during during the the proc processess of of data data collection collection includedincluded balancing balancing the the collection collection time time against against data data quality quality,, mitigatingmitigating thethe effectseffects of of internal inter- nalreflections, reflections, and and ensuring ensuring that that the the plane plane of of data data collection collection is properlyis properly leveled. leveled. MostMost gemstones gemstones cannot cannot be be cooled cooled to to liquid liquid nitrogen nitrogen temperatures temperatures to to optimize optimize the the resultsresults from from PL PL spectroscopy. spectroscopy. Diamonds Diamonds are are the the exception exception as as they they have have a alow low thermal thermal expansionexpansion coefficient coefficient and and extremely extremely high high thermal thermal conductivity, conductivity, so so they they can can be be cool cooleded to to lowlow temperatures. temperatures. Other Other gems, gems, if ifcooled, cooled, have have a amuch much higher higher risk risk of of fracture. fracture. For For example, example, thethe coefficient coefficient of of thermal thermal expansion expansion of of corundum corundum is is five five times times greater greater [9] [9 and] and the the thermal thermal conductivityconductivity is isat at least least 65 65 times times lower lower than than diamond diamond [10] [10. ]. Therefore,Therefore, since since diamonds diamonds can can be be cooled cooled,, PL PL maps maps are are typically typically conducted conducted with with the the ◦ stonestone immersed immersed in in liquid liquid nitrogen nitrogen (− (196−196 °C)C),, although although using using a cryostat a cryostat is isalso also a possibility
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