The look: An architectural detail used purely for decoration by American colonists took on a practical purpose in Victorian-era homes: concealing the holes required to retrofit hanging gaslighted fixtures. They add texture and shadow lines, but even by today's standards plaster medallions are a luxury, and they need to be installed by a skilled craftsman, which can be costly.
The shortcut: Polyurethane medallions made from molds have all the detail of plaster but very little of the weight. Installing one—either for decoration only or to hide an electrical box—requires just a few beads of construction adhesive.
Shown: A medallion with a pronounced leaf detail (small) suits an Italianate or a Second Empire home. For a Greek Revival, cove circles (medium) or flatter acanthus leaves (large) make a better match with the interior molding. $13–$65; Ekena Millwork
Tip: To replicate the texture of real plaster, mix joint compound with water until it reaches a pancake-batter consistency, then brush it onto the medallion before installing it.