In the sides, cover and base are panels of the fragile and exquisite translucent enamel made using plique-à-jour, a difficult enamelling technique which involves firing the glass into cells and removing the backing to create an effect like stained glass. When the cup is placed on a shaft of light it appears to be set with miniature stained-glass windows.
Pieces decorated with this sort of enamel are known from inventories of people such as the great fourteenth-century patron John, Duke of Berry, brother of king Charles V of France. This cup, however, is the only one to survive from this period. https://en.wikipedia.org/wiki/Mérode_Cup
(example)
Wow…!