Garnets, worked in the cloisonné technique, featured prominently in the luxury jewelry of the Franks. Jewelers would solder small compartments arranged in geometric patterns onto the surface of a metal disk. In those cells, or cloisons, they would place a textured piece of gold foil, which would show through the thin translucent garnet that would then be set on top.
J. Pierpont Morgan, London and New York (until 1917)
Ricci, Seymour de. Catalogue of a Collection of Gallo-Roman Antiquities Belonging to J. Pierpont Morgan. Paris: C. Berger, 1911. no. 12, p. 7, pl. II.
Brown, Katharine R., Dafydd Kidd, and Charles T. Little, ed. From Attila to Charlemagne: Arts of the Early Medieval Period in the Metropolitan Museum of Art. New York and New Haven: The Metropolitan Museum of Art, 2000. p. 238, 344, fig. 20.20.