Analysis of Selected Gemstones

Laboratory 5

Analysis of Selected GemstonesPrimary Investigator: Sumita Ghosh

Co-Investigator: Hi Vo

Lab Section Number 103AExperiment Conducted 2nd April 2015

Report Submitted 9th April 2015

Sumita Ghosh, Hi Vo 2

1 Executive Summary

1.1 Overview

Gemstones are valued for their natural beauty, and often their interesting colors. In this laboratoryexperiment gemstones were analyzed for first their genuineness and second their chemical compositionwith respect to color to see how they correspond. In doing so it was discovered that silicon atoms arehighly reflective and that microscopic grain boundaries are not the same as macroscopic color boundaries.Macroscopically smooth boundaries may be very rough microscopically, though more work is needed toverify this observation.

1.2 Methodology

Four gemstones were chosen to experiment on: amethyst and sodalite (to be crushed to fine powderand scanned with the x-ray diffractometer) to make a preliminary judgement of the genuineness of thegemstones, and tigers eyes and unakite (to be gold-coated on top and painted with conductive silverpaste on the bottom in order to be scanned with the scanning electron microscope and energy dispersivespectrometer) for chemical analysis.

The diffractometer ran at 30 kV and 15 mA, and the electron microscope ran at 15 kV and its defaultsettings given by [1]. The energy dispersive spectrometer was used to take point and line scans at everylocation where an image was taken with the scanning electron microscope.

1.3 Findings and Implications

The gemstones are genuine and their colors may have many different causes. Key impurities that deter-mine the color of the gemstones are in such low quantities that they cannot be detected by the x-raydiffractometer or the energy dispersive spectrometer - iron in amethyst (diffractometer) and sodium inthe tigers eyes or potassium in the unakite (spectrometer). This implies that an infinitesimal amount ofan impurity can drastically alter the appearance of the gemstone. Other potential causes of gemstonecolor include crystal structure and defects within that structure.

2 Introduction

2.1 Motivation

We wished to validate the identity of different gemstones and understand where their colors come from.Through this we wished to gain experience formulating and executing our own experiments using an x-raydiffractometer and a scanning electron microscope attached with an energy dispersive spectrometer.

2.2 Background

Natural gemstones are valued for their beautiful colors, which arise from defects within the material [2].Defects include vacancies and contaminants in otherwise colorless materials such as diamond, quartz,and fluorite. Gemstones are always crystalline, so their composite molecules form a crystal structurethat can be detected on an X-ray diffractometer [2].

The gemstones used in this lab were bought on Amazon.com [3], so before learning about the colorsof the gemstones, it was necessary to verify the identity of the crystals. This was done using the RigakuMiniFlex R©II x-ray diffractometer and comparing the resulting spectra to the known PDF files in thePDF-4+ database [4].

In an X-ray diffractometer (XRD), the source and detector(s) are on opposite sides of the sample,which sits on a stage between them. The stage and detector rotate simultaneously (the latter rotatestwice as much as the former) so that ”the angle of incidence (θ) onto the specimen is always equal tothe angle of scattering (θ) into the detector” [5]. Those angles are consistent because both the sourceand detector(s) have thin slits in front of them which ”define and collimate the incident and diffractedbeams” [6] - the detectors even have a setup that ensures the beam is focused into the slit before enteringthe actual detector. This consistency allows for greater precision in the diffraction peaks of the emissionspectrum.


Once the diffraction peaks are known, Bragg’s law can be used to find the corresponding d-spacings.Bragg’s law is given by

nλ = 2d sin θ (1)

The d-spacings can be paired with their respective intensities and inputted into the PDF-4+ database[4] to find the materials that have peaks closest to the peaks measured in the XRD. One software thatmakes this searching process much easier is the SIeve program. The method to use it is detailed in [7].

Once the identity of the gemstones is verified, the color can be analyzed to see how the patterns changewith chemical composition. This can be done using a scanning electron microscope (SEM) attached withan energy dispersive spectrometer (EDS).

The scanning electron microscope is an instrument that accelerates electrons from a usually thermionicsource with a high-voltage transformer system and then focuses the electrons with electromagnetic lensesinside a vacuum. The electrons are then scattered by the (bulk) sample and detected by electrondetectors, which gives enough information to create an image (by taking the diffraction pattern andforming its reciprocal space) and emission spectrum of the sample (using the EDS) [8]. Because theSEM bounces electrons off of the sample, the sample is required to be electrically conductive. If it isnot conductive, the sample must be coated in a conductive material (such as carbon or gold particles)so that it can be seen by the SEM and EDS.

An EDS shoots a beam of electrons through layers of electromagnetic lenses to focus them onto thesample, which then bounce off the sample and into a detector (as shown in Figure 1). The detector thencounts the number of x-ray signals for every wavelength.

These counts are plotted against the voltage of the beam (which corresponds to wavelength) andpeak-fitting is done automatically in the EDS software to approximate the most probable composition ofthe sample. This fitted composition may be false if the material contains rare or heavy elements becausethe software generally chooses more common elements, even over rarer elements that may fit better. Thisis because the EDS software is optimized for the most common cases, so for those the fitted compositionsare usually correct [9].

Figure 1: Simple diagram of an EDS set-up.

The SEM used in this lab was the Hitachi R©TM-3000 equipped with a Bruker R©Quantex 70 EDS.The TM-3000 contains a backscattered electron detector which records electrons scattered at high anglesand a Bruker Nano X-Flash R©430-H detector which analyzes x-rays.

Unfortunately the XRD requires that the samples put into it be either highly polycrystalline orpowdered in order to output a full spectrum, and our purposes require that the SEM and EDS look atthe bulk sample in order to look at the coloring. It is very difficult and time-consuming to powder onlypart of a gemstone, and time restrictions prevented crushing the sample after analyzing it in the SEM.Thus four samples were used: two samples were crushed for the XRD to verify the genuineness of thegemstones, and the other two samples were gold-coated to be conductive enough to be seen in the SEMso they could be analyzed for their color properties.

3 Experimental Procedure

In order to validate the identity of the gemstones used, first the XRD spectra had to be matched upwith the recorded gemstone spectra from PDF-4+. In order to take the XRD spectra, the gemstoneshad to be crushed into a powder, so the same gemstone that had its XRD spectra taken could not alsobe analyzed by the SEM and EDS. Therefore the assumption was made that once multiple gemstones


were verified as genuine, the other gemstones could be assumed to be genuine, and this would be checkedby looking at the color and chemical composition of the gemstones.

3.1 Preparation

The two samples to be crushed and verified were chosen to be amethyst and sodalite - the former becauseit is basic quartz and therefore likely easy to identify, and the latter because it had its own very complexdiffraction pattern in the PDF-4+ database. The samples were each crushed into fine powders withmortar and pestle.

The two samples chosen for SEM and EDS analysis were tigers eyes and unakite because they bothhad interesting color patterns - the former has a subtle brown pattern and the latter is very contrastedwith red and green. These were gold-coated on top and painted with silver on the bottom and sides sothat the whole sample was covered with a conductive material. This coating is shown in Figure 2.

Figure 2: The tigers eyes and unakite gemstones were coated like this. Pictured here is the coated tigerseyes.

3.2 Procedure

The powdered samples were put into the XRD and scanned from 3 degrees to 85 degrees at 30 kV and15 mA (standard for our lab) at 2 degrees a minute. After the scans were done the peaks were indexedby first finding the powder diffraction file from the PDF-4+ database using the SIeve software [7] andthen matching each measured peak to its closest peak in the powder diffraction file.

The bulk samples were placed into the SEM and pictures were taken of two or three regions on thesurface of each sample. After the pictures were taken, a point scan was done using the EDS in order toobtain average compositions of each region along with composition maps showing relatively how muchof each element is in each region compared to the other elements. Finally a line scan was taken to showthe specific counts of each element’s signals across a color boundary on the gemstone to compare eachcolor and its composition.

4 Results and Discussion

4.1 X-Ray Diffraction

As stated in sections 2.2 and 3, this portion of the laboratory was conducted in order to verify thelegitimacy of the gemstones. It is assumed that the four gemstones bought together from Amazon havethe same quality, as they came in the same collector’s box.

4.1.1 Amethyst

Amethyst is known to be made of silicon dioxide naturally doped with iron [10]. The defects cause byiron are what give amethyst its purple hue [2]. If the amethyst acquired through Amazon were genuineamethyst, it would have peaks that match up with silicon dioxide, and perhaps have smaller peaks thatmatch up with the iron. As Figure 3 shows, the first part of that statement is correct - the peaks domatch up quite well with SiO2, though from table 1 it is seen that there can be an error of up to 12%in the calculated d-spacings. However the second part is not - iron does not appear in its pure form atall in the crystal structure. Figure 4 shows that not a single peak in the range 2θ = 35◦ to 85◦ matchesfor the measured amethyst and the recorded iron. In fact, when searching through the database foriron compounds that may match with the measured diffraction spectrum of amethyst, none were evenremotely close. This means that if iron does form its own pockets in amethyst, it does so in a different


phase from pure iron. However it is more likely that iron forms substitutional defects [10] and thus onlychanges the lattice parameter slightly. This is reflected in the spectrum as many of the peaks measuredhad related d-spacings that were slightly larger than the recorded d-spacings from the powder diffractionfile of silicon dioxide, as shown in table 1.

Figure 3: XRD spectrum of powdered amethyst.

Figure 4: XRD spectrum of powdered amethyst overlayed on top of the PDF-4+ file of iron, from2θ = 35◦ to 85◦. As the Figure shows, there is no commonality between the iron and the amethyst inthis range. It is possible that there may be another phase of iron that has peaks in common with thisspectrum, but such a peak was not found when using the SIeve program on the PDF-4+ database inKresge Library.

Figure 5: The amethyst gemstone before it was crushed into a powder.


Table 1: Peaks from the amethyst XRD spectrum. These were put into the PDF-4+ SIeve program inorder to find the best fitting material that this could be. The SIeve program takes in the d-spacingsand relative intensities of the XRD spectrum, so those were calculated as well and are shown here. Theclosest approximation, with d-spacing consistently within 1

8 of tabulated values, was SiO2 — the mainmaterial of amethyst[2]. This information, combined with the outer appearance of the original gemstoneshown in Figure 5, suggests that the amethyst is genuine.

2 θ d rec. d error intensity rel. intensity h k l20.82 4.26297 4.25499 0.002 7037 19 1 0 026.60 3.34832 3.34347 0.001 36240 100 1 0 136.50 2.45967 2.45687 0.001 2870 8 1 1 039.42 2.28394 2.28149 0.001 3288 9 1 0 240.28 2.23714 2.23613 0.000 1707 5 1 1 142.42 2.12910 2.12771 0.001 3230 9 2 0 045.74 1.98198 1.97986 0.001 1763 5 2 0 150.10 1.81923 1.81796 0.001 6963 19 1 1 255.30 1.65984 1.67173 0.007 2020 6 2 0 259.90 1.54289 1.65919 0.070 4395 12 1 0 364.00 1.45358 1.60827 0.096 698 2 2 1 067.74 1.38214 1.54153 0.103 5705 16 2 1 168.28 1.37252 1.45289 0.055 4242 12 1 1 373.46 1.28800 1.41841 0.092 720 2 3 0 075.64 1.25620 1.38210 0.091 2313 6 2 1 277.62 1.22903 1.37496 0.106 902 2 2 0 379.78 1.20109 1.37188 0.124 2893 8 3 0 181.44 1.18075 1.18017 0.000 1957 5 3 1 083.82 1.15318 1.15298 0.000 1660 5 3 1 1

4.1.2 Sodalite

Sodalite was also powdered, in the same way as the amethyst - using mortar and pestle. The spectrain Figure 6 shows that the peaks are almost all perfectly aligned. This is further corroborated in table2, which shows that the error of the calculated d-spacings compared to the recorded d-spacings in thePDF-4+ database is, except for one peak, less than 2%. That peak may be off by more because ofdefects — gemstones tend to have many [2] — or because of contaminants that were introduced duringthe measurement. This does imply that the sodalite is genuine, and because both the sodalite and theamethyst are suspected to be genuine, the tigers eyes and the unakite may also be assumed to be genuine.This will be corroborated with knowledge of their chemical composition and their coloring.

Figure 6: (a) The PDF-4+ file for sodalite and (b) the file overlayed with the measured spectrum fromthe sodalite in the XRD. They are very similar with almost the same peaks.


Table 2: Peaks from the sodalite XRD spectrum. The calculated d-spacings and their respective relativeintensities were put into the PDF-4+ SIeve program in order to find the best fitting material that thiscould be. The closest approximation was sodalite itself, with highest error less than .12%. This impliesthat the crystal is genuine, as its appearance (Figure 7) suggests.

2 θ d rec. d error intensity rel. intensity h k l14.20 6.23196 6.28403 0.008 8355 42 1 1 017.86 4.96226 4.44348 0.117 6933 35 2 1 024.68 3.60428 3.62809 0.007 20060 100 2 1 128.08 3.17511 3.14201 0.011 4193 21 2 2 029.74 3.00156 2.81030 0.068 2742 14 3 1 031.98 2.79624 2.56544 0.090 2608 13 2 2 235.14 2.55169 2.46480 0.035 6207 31 3 2 038.04 2.36356 2.37514 0.005 5007 25 3 2 143.36 2.08509 2.09468 0.005 7313 36 4 1 145.80 1.97952 1.98718 0.004 1127 6 4 2 058.94 1.56571 1.57101 0.003 2275 11 4 4 060.94 1.51903 1.52410 0.003 1298 6 5 3 062.88 1.47675 1.48116 0.003 2262 11 6 0 064.80 1.43755 1.44166 0.003 1935 10 6 1 179.34 1.20665 1.20936 0.002 1742 9 7 2 1

Figure 7: The sodalite gemstone before it was crushed into a powder.

4.2 Scanning Electron Microscopy

4.2.1 Tigers Eyes

The tigers eyes is made of silicon, oxygen, iron, and sodium atoms - it is essentially quartz doped withcrocidolite [2] just as amethyst is quartz doped with iron. However, as Figures 8 and 11 show, there isso little sodium that the EDS does not even pick it up. That makes sense because crocidolite’s chemicalformula is Na2(Fe+3

2 Fe+23 )Si8O22(OH)2, which has five iron atoms for every two sodium atoms, and there

is already a low concentration of iron - less than 6% can be detected for both regions measured here. Butthis does corroborate the idea that this tigers eyes gemstone is genuine, as the elements are the same aswhat should be detected in tigers eyes naturally.

As the line scans in Figures 8a and 11a show, the change in shade does correspond to a change incomposition. But this is expected - the SEM gives different shades depending on the different ways theelectrons react to the surface, which is dependant on the elements on the surface.

If there is any way that the shades of the SEM picture can match the shades of color on the actualgemstone, the grain boundary would actually be distinct from one region to the next and the grainsize would be enormous because the patches on the surface of the gemstone are large enough to beseen macroscopically, as shown in Figure 14. Even if the different color patches were made of multiplegrains, if they have a different chemical composition then there would likely be a region that could bephotographed on the SEM such that half of the image is one shade and half of the image is anothershade. This is not necessary, but it is the only way that color could occur that is verifiable using thetools available for this laboratory. From the information gathered here, however, it seems there is noverifiable way that the color can change in this tigers eyes gemstone.


Figure 8: (a) The tiger eyes gemstone’s spectra across the surface, measured by an EDS line scan, and(b) the corresponding compositional bar chart measured using a point scan at the pictured image (atthe top right of b).

Figure 9: Compositional maps of the tigers eyes surface from which the line and point scans were taken(Figure 8). (a) The region of the map. (b) Concentration of iron. (c) Concentration of oxygen. (d)Concentration of silicon.

Figure 10: Spectrum from the line scan of the region in Figure 8a.


Figure 11: (a) The tiger eyes gemstone’s spectra across the surface, measured by an EDS line scan, and(b) the corresponding compositional bar chart measured using a point scan at the pictured image (atthe top right of b).

Figure 12: Compositional maps of the tigers eyes surface from which the line and point scans were taken(Figure 11). (a) The region of the map. (b) Concentration of iron. (c) Concentration of oxygen. (d)Concentration of silicon.



Figure 14: The surface of the tigers eyes gemstone that was looked at under SEM. The subtle changes incolor were not reflected in the changes in shade in the SEM - nowhere is there a line separating a largeregion of one shade from a large region of another. Figures 9 and 12 only show small domain boundaries,but from this image it is evident that if the differences in color could be seen on the SEM, they wouldbe much larger than the domains that were actually seen.

4.2.2 Unakite

As with the tigers eyes gemstone, the unakite sample was also measured with an SEM and its attachedEDS. Unakite is made of orthoclase (KAlSi3O8), epidote (Ca2(Fe+3,Al)3(SiO4)3(OH)), and quartz (SiO2)[11]. This means that genuine unakite has some signals for potassium, aluminum, silicon, oxygen, calcium,and iron. Figures 15 and 18 show that the composition does include the last five in that list, but notthe first. However only orthoclase contains potassium, and it only has one atom of potassium for everyeight atoms of oxygen it has - so the amount of potassium may be negligible, and it is still highly likelythat the unakite is genuine.

Figure 21 shows how much contrast the surface of unakite has - it is all red and green. The colorsshould come from the different minerals it is made of - so ideally this would be verified by EDS. However,finding a color boundary is difficult when the shade is not necessarily related to the color change, asfound in Section 4.2.1 with the tigers eyes gemstone.

Figures 15 and 18 show something interesting. Although the line scans both show silicon dominatingover all other elements in the small region of the line scan, the overall point scan contains much moreoxygen (as shown by the bar charts). Looking back at the composition of unakite, it seems that all threeminerals unakite is made of have more oxygen than silicon, so this means that silicon reflects electronsmuch more than oxygen does — about twice the amount of oxygen give only half the counts that silicondoes.

Figure 15: (a) The unakite gemstone’s spectra across the surface, measured by an EDS line scan, and(b) the corresponding compositional bar chart measured using a point scan at the pictured image (atthe top right of b).


Figure 16: Compositional maps of the unakite surface from which the line and point scans were taken(Figure 15). (a) The region of the map. (b) Concentration of aluminum. (c) Concentration of silicon.(d) Concentration of calcium. (e) Concentration of oxygen. (f) Concentration of iron.


Figure 18: (a) The unakite’s spectra across the surface, measured by an EDS line scan, and (b) thecorresponding compositional bar chart measured using a point scan at the pictured image (at the topright of b).


Figure 19: Compositional maps of the unakite surface from which the line and point scans were taken(Figure 18). (a) The region of the map. (b) Concentration of iron. (c) Concentration of oxygen. (d)Concentration of silicon.


Figure 21: The surface of the unakite gemstone that was looked at under SEM. Figures 16 and 19 onlyshow small domain boundaries, but from this image it is evident that if the differences in color could beseen on the SEM, they would be much larger than the domains that were actually seen. Alternatively,if the color boundaries are very jagged microscopically and the roughness of the boundaries cannot beseen by only the human eye, then it is possible that the boundary was just not photographed becausethere is no region where a full half of the screen would be filled with one shade and the other half withanother. This is shown better in Figure 22, where there is a clear large region where the shade darkensbut it is not a straight line.


Figure 22: There is a clear large region where the shade darkens at this region on the surface of theunakite. It is possible that the region extends into a rough boundary with many peninsulas like thisone, but it is also possible that this is only a larger grain but still does not correspond to the externalmacroscopic coloring.

5 Conclusion

This collection of gemstones were almost certainly genuine, with only sodium and potassium not foundin the tigers eyes and unakite respectively, and a slight (12% error) shift in the expected spectra ofthe amethyst and sodalite. A more rigorous identification procedure for the amethyst and sodalitewould include calculations and comparisons of lattice parameters, and future work could include suchcalculations.

The results from learning about color are not as clear. The tigers eyes sample did not show anysignificant difference in chemical composition across visible grain boundaries, and there were no clearboundaries in the SEM images that seemed to correspond to the macroscopic color patterns on thesurface of the sample. It is possible that the tigers eyes’ color changes happen because of the mineral’snatural polycrystalline nature (as most minerals do not grow in a single crystal). Future work wouldtake an XRD spectrum of a bulk tigers eyes sample, preferably from the same vendor on Amazon, andcompare the intensities of the peaks to see if there is a preferred orientation of the structure.

The unakite did show a little bit of difference in the chemical composition based on what weresuspected to be color boundaries, but they were not able to be verified because the boundaries wovein and out rather than being straight as the color boundaries seem to be macroscopically. From thedata gathered it does appear that the different colors can be caused by differences in local chemicalcomposition, but this was not able to be verified conclusively. Future work would find a way to mark


the color boundaries of the sample prior to entering the SEM in such a way that they could be seenthrough the SEM. This could possibly be done with a diamond scribe drawing dashed lines on the colorboundaries.

Three of the four gemstones chosen for this laboratory happened to be made of primarily quartz. Thetwo gemstones chosen to do SEM with were made of significantly more oxygen than quartz (about 20%more for tigers eyes and almost twice as much for unakite), yet both of them showed many more countsfrom silicon than oxygen during the line scans. This implies that, for an electron beam, silicon is aboutfour times as reflective as oxygen. Other elements key to the color of the gemstone were undetectableby the machines used. The iron that makes amethyst purple was not found in its diffraction spectrum,and the EDS could not find any sodium in the tigers eyes or potassium in the unakite. This means thatthe slightest change in composition can drastically alter the appearance of a gemstone - an infinitesimalimpurity can affect the coloring of the mineral.

References

[1] Hitachi, Hitaci Tabletop Microscope TM3000, http://hitachi-hta.com/sites/default/files/literature/TM3000-TableTopSEM-BrochureHTD-E188Q.pdf, 4/9/2015.

[2] K. Nassau, The physics and chemistry of color: the fifteen causes of color, Wiley series in pure andapplied optics, Wiley, 2001.

[3] Geocentral, Gemstone Collection Box, http://www.amazon.com/GeoCentral-NGEM-Gemstone-Collection-Box/dp/B006WREUQU/ref=sr 1 6?ie=UTF8&qid=1427671384&sr=8-6&keywords=rocks, 3/29/2015.

[4] I. C. for Diffraction Data, Powder Diffraction File, Kresge Engineering Library, UC Berkeley, 2015.

[5] L. W. Martin, Laboratory 3: Precision X-ray Diffraction, 2015.

[6] B. Cullity, Elements of X-ray Diffraction, Addison-Wesley series in metallurgy and materials,Addison-Wesley Publishing Company, 1978.

[7] L. W. Martin, Homework 8, 2015.

[8] L. W. Martin, Chapter A: Electron-based Microscopy and Diffraction Intro to SEM, TEM, andMore, March 9th 2015.

[9] C. P. Barrett, conversation, 3/3/2015.

[10] F. Di Benedetto et al., Physics and Chemistry of Minerals 37, 283 (2010), A Fe K-edge XAS studyof amethyst.

[11] R. K. Pabian and A. Cook, (1976), Minerals and Gemstones of Nebraska: A Handbook for Studentsand Collectors.

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Analysis of Selected Gemstones

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