Amongst the surviving gold-glass fragments or fondi d'oro the depiction of the cross on its own is unusual although many do have Christian symbolism. The majority of these fragments with chipped edges appear to have been found embedded in the mortar which closed the opening of each loculus or wall-tomb within the catacombs. It has been suggested that they were used as identification marks for the mourning relatives. Cf. D. B. Harden, Glass of the Caesars, exhibition catalogue, Milan, 1987, pp. 277-286, for other surviving examples showing this gold-glass technique.