The Pace di Siena and its Gems

The Pace di Siena is a unique en ronde bosse enamel, with few comparable pieces in Europe. The enamel, preserved and exhibited in the Arezzo Diocesan Museum (Italy), is surrounded by a frame decorated with thirty blue and pink gems whose identity has never been explored through analytical studies. The historical art literature has traditionally described these gems as tourmaline, aquamarine, beryl, and quartz, without delving into their provenance. Only a few hypotheses exist regarding the manufacture of the precious frame. Gemological analysis, in situ Raman spectroscopy, and energy-dispersive portable x-ray fluorescence (ED-pXRF) analysis have, for the first time, revealed the true nature of the gems, identifying them as fourteen natural blue corundum (sapphires) and sixteen natural pink spinels. The analysis of the inclusions also supports the hypothesis of heat treatment in the natural sapphires while suggesting a possible reuse for the natural untreated spinels, as indicated by the presence of distinctive holes. Additionally, semiquantitative geochemical data obtained by ED-pXRF after calibration through particle-induced x-ray emission (PIXE) analysis on a set of reference gemstones has provided valuable insights into the possible origins of the sapphires (which seems to have a metamorphic origin consistent with a possible Sri Lankan origin) and the pink spinels (which may have originated be come from Myanmar or Tajikistan). The study highlights the intersections between the materiality of the object, the concept of object biography, the provenance of their gems, and its chaîne opératoire.