MINERALOGY AND HISTORY OF SEMI-PRECIOUS GEMSTONES ON THE BORGHESE-WINDSOR CABINET (Invited Presentation)

The Borghese-Windsor Cabinet at the J. Paul Getty Museum is a piece of furniture exquisitely decorated with agate, lapis lazuli, and other semi-precious stones and built around 1620 for Camillo Borghese (later Pope Paul V). It is traditionally assumed that all agate gemstones acquired during the 1500s to 1600s were sourced from the Nahe River Valley near Idar-Oberstein, Germany. Agate from Brazil (a major global producer) was imported into Germany by the 1800s. However, it is possible that some agates were imported in the 1700s or earlier. A primary research goal was to determine if the agates in the Borghese-Windsor Cabinet are of a single origin.

Agates are composed of silica, mainly as the mineral quartz, but also as metastable moganite. Both quartz and moganite will crystallize together as the agate forms, but moganite will convert to quartz over tens of millions of years; thus, relatively older agate contains less moganite, and this ratio can be measured with a variety of analytical techniques.

The agate gemstones of the Borghese-Windsor Cabinet cannot be removed for detailed Raman or XRD mineralogical analyses. Because of this, we first analyzed agate specimens of known provenance from the collections of the Natural History Museum of Los Angeles County (NHMLAC) and the Smithsonian Institution National Museum of Natural History (NMNH) using three different techniques: Raman mapping, micro-XRF mapping, and visible-shortwave infrared hyperspectral imaging (HSI). After correlating Raman/XRF and HSI data, we performed in situ HSI of the entire cabinet. From these data, it was determined that the much of agate on the cabinet is likely from Idar-Oberstein, but a significant portion was either originally sourced from a different locality, or possibly replaced at a later date.