Coloured glass coating of quartz beads before the 13th century: An undocumented sapphire imitation

Abstract The Abbey of Saint‐Maurice d'Agaune in Switzerland exceptionally accorded access to its treasure pieces for conservation purposes. The restoration of the Great Shrine of Saint Maurice (13th century) gave us the opportunity to examine the gemstones set into this remarkable object made with older replacements. Among sapphires, emeralds, garnets, pearls, glasses and others, we discovered two pierced quartz beads coated with blue‐coloured glass. Microscopic observations, Raman and chemical analyses led us to determine the nature of this cobalt‐coloured, lead‐rich, silica glass covering the natural quartz beads. To the best of our knowledge, these two coated beads are the oldest examples of glass coating on a gemstone for jewellery purposes. They raise numerous questions about the technique employed to produce this exceptional coating, giving two beads which thus look like sapphire. This technique combines advanced glasswork, glazing technics and gemmological knowledge.