This is a video produced by The British Museum which demonstrates the metalworking technique called repoussé. Metal sheets are hammered into shape from behind to create the ornamentation found on medieval reliquaries.
While I'm on break from making art, I'll try to post some art history videos. There are some interesting ones by Smarthistory which has partnered with Khan Academy to offer free art history courses online . This video discusses the metal cover for the Lindau Gospels from the Carolingian period, the influence of Classical Roman art on early Medieval art, and the symbolism of the composition and jewels which allude to the book of Revelation: