CHARACTERIZATION OF GEMSTONES USING ED-XRF TECHNIQUE

Introduction:

The energy dispersive X-ray fluorescence analysis (ED-XRF) is a well-established tool for multi-element composition analysis in different fields. It works non-conducting and it is non-destructive. No particular sample preparation is necessary if the specimens have an almost planar and smooth surface. For the characterization of precious gemstone material it is also important that the incident X-ray radiation dose is definitely too weak for inducing colour alterations, in contrast to WD-XRF. This is essential for gemstone testing. The concentrations of the major and minor and the trace elements give important hints for the type and/or the authenticity of the gemstone. Identification and quantification of such elements may allow tracking an emerald as an example down to its location of origin. Similarly the presence of certain trace elements also helps to distinguish between a valuable naturally formed gemstone (e.g. ruby) and a quasi-worthless synthetic crystal (e.g. synthetic ruby).

Instrumentation:

The Thermo Scientific ARL QUANT’X EDXRF spectrometer has been used for the non-destructive analysis of gemstones. Its lithium drifted silicon Si(Li) detector has a thickness of  3 mm which translates into better detection efficiencies for higher energy In addition, the direct excitation geometry and adjustable X-ray beam collimation allows analysis with small analysis spots while retaining most of the analytical sensitivity. X-ray beam collimators of various sizes are available to adjust the spot size (Fig.1). The sample imaging CCD camera allows positioning of small gemstones for efficient excitation and analysis.

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Table 1 shows the set of different excitation conditions used for the analysis of different Gemstone materials. The total counting time per analysis is less than 10 minutes. All measurements are performed in vacuum.

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Table 1

Gemstones are analyzed as such not to damage the sample in any way (non destructive). Small gemstones are placed in an XRF cup sealed with a 4 μm thick polypropylene film. A Fundamental Parameter (FP) approach using calibration with pure material has been used to perform the calibrations. This method is included in the standard quantitative package of the ARL QUANT’X and is known as Uniquant Library Software. The WinFTM FP treatment calculates a best-fit of the theoretical spectra with the measured ones. The free parameters of this iterative fit calculation are the unknown concentrations. The mathematical best fit is the “theoretical” or “standardless” result. The treatment assumes a sample which consists of plan-parallel “layers”.

Results & Discussion:

A number of different gemstones like natural pearl, Red coral, Sapphire, Ruby and Ruby with diffused silica were analyzed using the ED-XRF. All the analyses have been performed using variable X-ray beam collimators. The following figures show spectrum and results for the Gemstone analyzed so far at the laboratory.

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Conclusion:

Gemstones have been characterized by using non destructive ED-XRF analysis. A straightforward calibration using pure elements or compound (available from instrument library software) has been used to assess the gemstones . As such, the ARL QUANT’X proves to be a cost efficient and truly non-destructive analysis tool for the gemological lab. Next to pearl, coral, rubies and sapphires a similar approach can be implemented for the analysis of other precious stones such as spinels, chrysoberyls or emeralds.

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