In a back corner between the counterfacade and the campanile base is a short spirally ribbed stone column with a strange polished black stone on top shaped like a round loaf of bread. This is the lapis diaboli, and by tradition the Devil threw it at St Dominic while he was at prayer in the church and smashed the paving slab on which he was kneeling. The stone looks like an ancient Roman scale-weight examples of which have survived in other Roman churches.