Pearls are one of the most famous gems widely used for jewelry such as necklaces, rings, and earrings. While real pearls are grown inside mollusks, imitation pearls are entirely artificial objects created to imitate the real ones. [1,2]
X-ray fluorescence (XRF) is a non-destructive elemental analysis method used for pearl origin estimation and authentic screening of pearls [3,4,5]. In particular, micro-XRF, an XRF with a micro-beam spot, is a method to perform elemental analysis on small gems, or gems attached to a piece of jewelry without removing it. [5]
In this application note, we compared real pearl earring with imitation pearl earring, and we demonstrated elemental analysis using HORIBA’s micro-XRF XGT-9000 to evaluate and differentiate them non-destructively.
Figure 1. Pearls
The XGT-9000 X-ray Analytical Microscope (Figure 2) is an energy dispersive X-ray fluorescence microscope (micro-XRF) with upper irradiation with micro-spot size and a motorized XYZ stage. The micro spot size is helpful to hit a limited surface, such as a spherical shape of a pearl, or a small gem attached to a piece of jewelry without needing to separate it from its surrounding decorative parts. The motorized XYZ stage can analyze multiple pearls on a sample tray.
Figure 2. (a) HORIBA XGT-9000 X-ray Analytical Microscope; (b) Schematic diagram of the internal optics of the XGT-9000; (c) Multiple sample setting on a sample tray.
We prepared two different pearl earrings. One is a pearl earring with a certificate document as a famous seawater pearl (called Earring-A below). The other is an imitation pearl earring (called Earring-B below). We didn’t disassemble or remove any parts of the earrings.
We placed the two earrings on a sample holder at once. We carried out the analysis under a “partial vacuum” condition, where a thin film separates the optics (under vacuum) and the sample chamber (under ambient). The partial vacuum condition helps reduce in reducing the moisture loss from conchiolin of pearls compared to the full vacuum condition. Also, pearls have an aragonite (CaCO3) crystalline structure in nacre layers [6], and the structure might cause diffracted X-rays, which could lead to undesired spectral interference with fluorescent X-ray peaks in a spectrum. For this reason, we used a primary X-ray filter to reduce the undesired phenomenon during the analysis. The detailed analysis condition is described in Figure 3.
Figure 3a shows a photo and an XRF spectrum of the seawater pearl, Earring-A. The spectrum shows big peaks of calcium (Ca), which is the main composition of a pearl. We also detected peaks of strontium (Sr) and no significant peak of manganese (Mn). The result was consistent with the composition trend reported in the same type of seawater pearls [3, 4].
Figure 3b shows a photo and an XRF spectrum of the imitation pearl, Earring-B. The spectrum shows big peaks of lead (Pb), which is an unexpected element in pearls, though small peaks of Ca Mn and Sr were detected. The presence of Pb was consistent with the composition reported as a possible material used in imitation pearls [7].
Figure 3. Sample photo and XRF spectrum (a) a pearl earring came with a certificate as a seawater pearl, and (b) an imitation pearl earring. The analyses were conducted under partial vacuum conditions using a 100 μm ultra-high intensity probe and with 50 kV voltage and auto current at the primary X-ray generator. A primary X-ray filter called Filter 2 was used for each analysis. The spectrum analysis time was 30 seconds per sample.
Thus, elemental analysis using HORIBA’s micro-XRF XGT-9000 could show different elements between the real seawater pearl earring and the imitation pearl earring in 30 seconds even without removing the decorative parts.
[1] Gemological Institute of America, “Pearl Gemstone | Pearl Stone | Pearl Gem,” GIA.edu. [Online]. Available: https://www.gia.edu/pearl (accessed Jul. 19, 2024).
[2] M. Manutchehr-Danai, Dictionary of Gems and Gemology, Berlin, Heidelberg: Springer, 2009.
[3] C. Zhou, E. Yazawa, and M. Dragone, “Large freshwater cultured pearls with atypical bead nuclei,” Gems & Gemology, vol. 60, no. 1, Spring 2024.
[4] K. Wada and T. Fujinuki, “Factors controlling amounts of minor elements in pearls,” Journal of the Gemmological Society of Japan, vol. 13, no. 1–4, pp. 3–12, 1988.
[5] HORIBA, “X-ray micro-analysis for pearl characterization in forensic science,” HORIBA XGT Application Note, XGT-07. [Online]. Available: https://www.horiba.com/int/scientific/applications/others/pages/x-ray-micro-analysis-for-pearl-characterisation-in-forensic-science/ (accessed Jul. 19, 2024)
[6] R. Ozaki, K. Kikumoto, M. Takagaki, K. Kadowaki, and K. Odawara, “Structural colors of pearls,” Scientific Reports, vol. 11, no. 1, Article 15445, 2021.
[7] J. Hanano, M. Wildman, and P. G. Yurkiewicz, “Majorica imitation pearls,” Gems & Gemology, vol. 26, no. 3, Fall 1990.
X-ray Analytical Microscope (Micro-XRF)
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