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 1913); Estate of J. Pierpont Morgan(1913–1917)
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. 347, fig. 7l.