Composite gems in medieval era: the art of counterfeiting before the sixteenth century

This study explores the evolution of gemstone treatments, with a particular focus on composite gems, which consist of two or three layers of natural gems or artificial materials glued together with adhesives and pigments to imitate genuine natural gems (e.g. doublets and triplets). While traditional treatments such as foiling and coating are well documented, composite gems are largely absent from ancient literature. Pliny is the earliest source to mention the practice of gem imitation, even though with some condemnation. After a considerable gap, the first description of doublets and triplets appear in Leonardi (1502) and Cellini (1568), with the first literature reference to composite gems on goldwork dated to the fourth decade of the fifteenth century. The analysis of a collection of goldsmithing artifacts, dating from the late thirtheenth to the fifteenth centuries and preserved in Tuscany museums, revealed—for the first time—the unexpected presence of composite gems alongside a variety of gemstones and treatment methods. Gemological analysis, including Raman spectroscopy, and the study of adhesives using pyrolysis gas chromatography coupled with mass spectrometry, have shed light on their manufacturing processes. The results suggest that composite gemstones, particularly doublets and triplets, have been systematically produced in specialized workshops since the thirteenth century. This production is likely to have coincided with advances in the technology of cutting precious stones and artificial glass. This research not only sheds light on historical gemstone craftsmanship but also opens new perspectives for studying the evolution of gemology, challenging previous assumptions about the timeline and context of composite gemstone production.