Analysis of Precious Stones Using Energy Dispersion X-Ray Fluorescence Spectroscopy (EDXRF)

The appraisal of precious stones typically involves an array of instruments, including various general gemstone appraisal tools such as the stereoscopic microscope and more specialized analytical instruments.1

These analytical instruments include the Fourier transform infrared spectrophotometer (FT-IR) – a device generally used to confirm the application of heat treatment or resin impregnated treatment - and the UV-Vis spectrophotometer (UV-Vis) – a device used to determine the presence or absence of coloring treatment.

An energy dispersive X-ray fluorescence spectroscope (EDXRF) also represents a vital, indispensable instrument offering rapid, non-destructive composition analysis; for example, EDXRF is able to definitively detect the lead used in the lead glass-filling treatment of gemstones.

The article summarizes the use of a Shimadzu EDX-8100 in the composition analysis of rubies, emeralds and Paraiba tourmalines – popular, commonly encountered and commercially available gemstones.

Investigations were conducted into the instrument’s ability to distinguish between natural and synthetic stones and its ability to identify the geographic origin of any natural stones present in the study.

Samples

A variety of natural and synthetic precious stones and rough gemstones were used. These are outlined in the table below.

Types and Origins of Precious Stones and Gemstones.

Figure 1. Types and Origins of Precious Stones and Gemstones. Image Credit: Shimadzu Scientific Instruments

Elements

The following elements were used: 11Na - 92U

Sample Preparation

Samples were set appropriately to enable direct X-ray irradiation before being measured.

Ruby: Measurement Results and Notable Differences

Figure 2 displays both qualitative and quantitative analysis results for Tanzanian natural stone and synthetic stone.

In the example presented here, it was impossible to detect a trace amount of gallium in the synthetic stone. The natural stone (Tanzania) also presented a lower chromium content than the synthetic stone.

This highlights the potential for natural stones and synthetic stones to be distinguished by these differences, but it should be noted that in some instances, elements originating from a catalyst may also be detected in synthetic stones.

Results of Qualitative Analysis/Quantitative Analysis of Rubies.

      [wt%]
Element Composition Natural stone (Tanzania) Synthetic stone
Aluminum Al2O3 99.395 99.439
Titanium TiO2 0.018 0.025
Vanadium V2O5 0.003 0.017
Chromium Cr2O3 0.255 0.477
Iron Fe2O3 0.324 0.009
Gallium Ga2O3 0.005
Zirconium ZrO2 0.035

Figure 2. Results of Qualitative Analysis/Quantitative Analysis of Rubies. Image Credit: Shimadzu Scientific Instruments

Emerald: Measurement Results and Notable Differences 

Figure 3 displays both qualitative analysis and quantitative analysis results for two types of Columbian and Zambian natural stones, as well as synthetic stone.

Emeralds’ color stems from the presence of chromium, iron and vanadium, with stones’ hues changing depending on the amounts of these elements present.

In the example presented in Figure 3, results from the Zambian stone confirm high contents of chromium and iron and low content of vanadium versus the Colombian stone.

Comparatively large amounts of magnesium, sodium and potassium were also detected in the Zambian stone, as well as trace amounts of rubidium and cesium. These differences are a key indicator in terms of distinguishing this stone from the Colombian stone.

The price of emeralds is typically determined by their transparency, hue and depth of color (color strength), but their geographic place of origin is also a key factor in their economic value.

Emeralds tend to display distinctive features based on their respective geographic origin; for example, Colombian stones exhibit a bright green color, while Zambian stones are characterized by high transparency. Colombian stones are generally more expensive.

Distinguishing deep green Zambian emeralds from Colombian emeralds can be challenging and typically require the use of elemental analysis to determine the stones’ geographic origin confidently.

In the example presented here, the synthetic stone was identified by detecting the presence of rhodium - likely originating from a catalyst - and the lack of sodium and magnesium in the detection results.

Results of Qualitative Analysis/Quantitative Analysis of Emeralds.

        [wt%]
Element Composition Natural stone (Colombia) Natural stone (Zambia) Synthetic stone
Sodium Na2O 0.879 2.335
Magnesium MgO 0.954 2.518
Aluminum Al2O3 18.033 15.028 21.252
Silicon SiO2 79.021 77.925 78.098
Potassium K2O 0.031
Calcium CaO 0.031 0.037
Vanadium V2O5 0.518 0.069
Chromium Cr2O3 0.506 0.829 0.560
Iron Fe2O3 0.051 1.136 0.075
Gallium Ga2O3 0.006 0.005 0.003
Rubidium Rb2O 0.002
Cesium Cs2O 0.084
Rhodium Rh2O3 0.012

Figure 3. Results of Qualitative Analysis/Quantitative Analysis of Emeralds. Image Credit: Shimadzu Scientific Instruments

Paraiba Tourmaline: Measurement Results and Notable Differences 

Figure 4 displays both qualitative and quantitative analysis results for the rough gemstone and the cut stone – both of these were produced in Brazil.

Results presented in Figure 4 confirm that the stones’ primary components are oxides of aluminum and silicon. The samples were also found to contain manganese and copper. These discoveries combined were sufficient to conclude that both samples were natural Paraiba tourmaline.

Paraiba tourmaline is produced in three areas (Brazil, Nigeria and Mozambique). The stones investigated here were concluded to be likely of Brazilian origin due to their copper contents exceeding 0.6 w% (CuO). Further detailed analysis was also performed using LA-ICP-MS.2

The results also showed that the cut stone included a high manganese content of 4 wt% (MnO) in contrast to the 0.5 wt% (MnO) content of the rough stone.

Stones typically exhibit a strong green hue where Mn content is high, but the hues of both stones appeared to be an identical neon blue when viewed with the naked eye (Figure 5).

To differentiate the samples, they were observed using an OLS5000 3D measuring laser microscope (Figure 6). Green areas were noted in the stones’ interior and are believed to be an influential factor in the high content of Mn.

Table 1. Measurement Conditions. Source: Shimadzu Scientific Instruments

. .
Instrument  EDX-8100 (EDX-8000)
Elements Na - U
Analysis group Qualitative analysis and quantitative analysis
Detector  SDD
X-ray tube  Rh target
Tube voltage  15 [kV] (Na-V), 50 [kV] (Cr-U)
Tube current  Auto [μA]
Collimator  3 [mm φ]
Primary filter, channel  Non [Na-P], #1 [Sr-Ba], #2 [S-V], #4 [Cr-Rb]
Atmosphere  Vacuum
Integral time 50 [s] × 4 Ch
Dead time  Maximum 30 [%]

 

Results of Qualitative Analysis/Quantitative Analysis of Paraiba Tourmaline.

      [wt%]
Element Composition Rough stone (Brazil) Cut stone (Brazil)
Sodium Na2O 2.235 2.894
Aluminum Al2O3 45.291 42.494
Silicon SiO2 49.379 47.285
Potassium K2O 0.040 0.092
Calcium CaO 0.516 0.822
Titanium TiO2 0.027 0.074
Manganese MnO 0.561 4.021
Iron Fe2O3 0.032
Copper CuO 1.767 2.054
Zinc ZnO 0.017
Gallium Ga2O3 0.043 0.027
Rubidium Rb2O 0.006
Lead PbO 0.021
Bismuth Bi2O3 0.087 0.217

Figure 4. Results of Qualitative Analysis/Quantitative Analysis of Paraiba Tourmaline. Image Credit: Shimadzu Scientific Instruments

Observation Images of EDX Samples

Figure 5. Observation Images of EDX Samples (: X-Ray Irradiation Range 3 mm φ). Image Credit: Shimadzu Scientific Instruments

Observation Image of Cut Stone (OLS5000).

Figure 6. Observation Image of Cut Stone (OLS5000). Image Credit: Shimadzu Scientific Instruments

Conclusion

The results presented throughout this article highlight the potential for the differentiation of natural stone and synthetic stone – and the determination of the geographic origin of natural stones – to be estimated by evaluating the types of elements present, the differences in their contents and any further trace detection.

A comparison was made between measurements acquired using only EDX and appraisals of gemstones using EDX measurement in combination with other analytical instruments (FT-IR, UV-Vis).

This comparison confirmed that use of EDX with other instruments resulted in improved accuracy.3

References

  1. Dr. Ahmadjan Abduriyim: The Science of Gemstones, ARK PUBLISHING INC.
  2. Ahmadjan Abduriyim, Hiroshi Kitawaki, Masashi Furuya, and Dietmar Schwarz: “Paraiba”-Type Copper Bearing Tourmaline from Brazil, Nigeria, and Mozambique: Chemical Fingerprinting by LA-ICP-MS
  3. Ahmadjan Abduriyim and Hiroshi Kitawaki: Applications of Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA- ICP-MS) to Gemology

Acknowledgments

Produced from materials originally authored by T. Nakao and H. Nakamura from Shimadzu Scientific Instruments Inc.

The authors on which this piece is based wish to express their deep appreciation to Dr. Ahmadjan Abduriyim, who is the Representative of Tokyo Gem Science LLC and Director of the GSTV Gemological Laboratory, for various advice in connection with gemology and gemstone appraisal.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.

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