The Origin of Ancient Gemstones Unveiled by PIXE, PIGE and μ-Raman Spectrometry

Abstract

Gemstones set on ancient jewels are invaluable witnesses from the past. To obtain these much appreciated items, Men had established long-distance trade routes, the extension and historical evolution of which represent a major archaeological issue. For these studies, chemical characterisation has proved to be very useful. First, the elemental composition directly provides the nature of the gemstone, like for example, SiO₂ for quartz, Al₂O₃ for ruby and sapphire, Be₃Al₂Si₃O₁₈ for emerald. Secondly, the trace element content of a gemstone, when compared to reference gems of known origin, may unveil its provenance. However, to perform the chemical characterisation of these valuable items, suitable analytical methods must be non-invasive (neither sampling nor dismounting of the gems from the jewels) and non-destructive (no damage to the gem). Ion beam techniques such as particle-induced X-ray emission (PIXE) or particle-induced λ-ray emission (PIGE) meet these specific requiremens. PIXE permits the measurement of all elements heavier than sodium with a sensitivity reaching the μg/g, while PIGE extends the range of elements to lighter ones (Li, Be, B, F). These totally harmless techniques are directly applied to the gem in air with a 20-μm spatial resolution, allowing selecting specific regions of the crystal. The identification of inclusions, a provenance criterion often used in gemmology, can be complementarily performed by Raman micro-spectrometry. Indeed, the inclusions are often located out of reach of the ion beam in the crystal. This paper presents the benefits of the combined use of external beam PIXE/PIGE and Raman micro-spectrometry. The use of these modern analytical techniques is exemplified by the study of ancient gems: rubies from a Mesopotamian statuette, emeralds and garnets set on Barbarian jewels dating from the Dark Ages.