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Non-destructive Authentication of Gemstones Using Micro-XRF

Introduction

Verification of the authenticity and quality of gemstones is necessary to assess the value of the object. Many gemstones can be synthesized to a significant size and remarkable quality. It is essential that authentication testing is non-destructive because any destructive testing of a potentially valuable gemstone will reduce the article's value.

Micro X-ray Fluorescence (micro-XRF) is a simple non-contact, and non-destructive technique for the authentication of gemstones. In this technique, X-rays from an X-ray tube are shone on the sample. This, in turn, excites the sample to emit characteristic fluorescent X-rays for each element. Fluorescent X-rays can be emitted from tens to hundreds of microns within the sample and provide important information about the material's elemental composition. Samples can be analyzed as is without coating or the use of other destructive sample preparation treatments. This is important for testing valuable gemstones.

Instrumentation

The instrument used for demonstration is an EDAX Orbis PC Micro-XRF Elemental Analyzer. It is equipped with a 50 W Rh X-ray tube and can analyze elements in air or vacuum mode. A motorized turret integrating video and X-ray optics allows coaxial sample view and X-ray analysis. This geometry optimizes sample targeting accuracy and eliminates shadowing of the X-ray beam. The sample is viewed with a 10x color CCD camera for coarse sample positioning and a 75x color CCD camera with 3x digital zoom for fine positioning under the X-ray beam. The equipped 30 µm ultra-high intensity polycapillary X-ray optic generates a high-sensitivity convergent micro-spot X-ray beam, while 1 and 2 mm collimators are used to create a homogeneous beam over a larger area. Primary beam filters are available at all X-ray spot sizes to improve detection limits or reduce diffractive noise. The large area Silicon Drift Detector (SDD) provides improved sensitivity and liquid nitrogen-free operation.

Example

Pearls are organic gemstones produced by oysters. Pearls are composed of layers of calcium carbonate surrounding some tiny seed particle that has irritated the oyster. The coatings of calcium carbonate are the oyster's way of protecting itself from the irritant. Traces of Sr can be found in the pearl since Sr is a natural lattice substitute for Ca, and Sr is found in low-level concentrations in seawater. Imitation pearls are usually made up of plastic with a color coating. Bismuth oxychloride is commonly used as a colorant to create a pearl-like effect. Therefore, we should expect to see Ca and Sr peaks in a micro-XRF spectrum from a natural pearl (wild or cultivated). Ca peaks should be absent in a spectrum from an imitation pearl, and characteristic peaks from colorants, such as Bi peaks, are expected.

Four pearl samples were used in this authentication study (Figure 1): (1) An earring, (2) a bead bracelet, (3) a bead necklace, and (4) a chain bracelet with beads. The samples were laid on the Orbis PC sample stage and analyzed in vacuum mode at 40 kV X-ray tube voltage. One spectrum was collected on a bead from each sample with the same X-ray tube current, amp time, and acquisition time. The 1 mm collimator was chosen to cover a relatively large area on the bead.

Pearl samples from left to right are 1) an earring, 2) a bead bracelet, 3) a bead necklace, and 4) a chain bracelet with beads.
Figure 1. Pearl samples from left to right are 1) an earring, 2) a bead bracelet, 3) a bead necklace, and 4) a chain bracelet with beads.

 

In the overlaid spectra of all the samples (Figures 2 and 3), samples 1 – 3 show tall Ca peaks and tiny Sr peaks, suggesting these pearls are natural. There are no Ca peaks in the spectrum from sample 4, and Bi peaks are observed from this spectrum only. These indicate imitation pearls, and the shimmery and pearl-like appearance is produced by bismuth oxychloride. The Cu peaks in the spectrum from sample 4 are presumably from copper oxides which are commonly used as pigments to produce various colors.

Overlaid spectra from sample 1 (red), 2 (cyan), 3 (orange), and 4 (gold).
Figure 2. Overlaid spectra from sample 1 (red), 2 (cyan), 3 (orange), and 4 (gold).

 

Overlaid spectra from Figure 2 with bumped-up peaks.
Figure 3. Overlaid spectra from Figure 2 with bumped-up peaks.

 

Conclusion

Micro-XRF can be applied to authenticate various potentially high-value gemstones. In this example, the Orbis PC Micro-XRF System was successfully used to authenticate pearls. Authentication was done fast and non-destructively since the X-ray beam does not leave any beam damage on the sample, and this technique does not require any potentially destructive pre-treatment of the sample. In the rapid analysis, authentication was readily achieved by determining the major and trace elements found in the natural pearls and elements from imitation pearls' color coating.