 They offer a foretaste of the bejeweled nature of the Celestial city, and particular types of gem were believed to have actual powerful properties in various "scientific", medical, and magical respects, as set out in the popular lapidary books.Many of the original gems and pearls are now lost, but there are replacements in paste or mother of pearl.The reliefs show the Four Evangelists with their symbols and background foliage in the compartments at top and bottom, and two figures each in four compartments on the sides.