The font is an ancient green basalt bath-tub on a polychrome marble plinth, itself on a circular polychrome marble base. It is in a circular depression which is a reminder of the original baptismal plunge-pool before the re-ordering by Pope Gregory XIII in the latter part of the 16th century. The void is surrounded by a balustrade with two entrances, one facing the public doorway and the other opposite, and these are flanked by ball finials. The polychrome marble cladding of the walls and floor here are by Borromini.