Ring Settings: Cellini's Closed Backs Hid Colored Foil
Six ring settings in cross-section: bezel, prong, closed back, pavé, channel and gypsy. In Cellini's 16th-century account, a closed back hid colored foil.
Picture credit
"Stirrup Ring" by Unknown maker — https://commons.wikimedia.org/wiki/File:Stirrup_Ring_MET_DP701987.jpg — CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/)