There could be more to it than the
religious meaning, but I'd bet that the frequent use of shell-shaped details on church decoration inspired decoration everywhere else while loosing or changing its sense. I suspect that lay representations of shells and pearls would have been nothing more than what sketches of a diamond are today - a little reminder of glamor or the jewelry trade. A pearl in (scallop) shell is an old fashioned lover's charm, and that sounds close to the carving you describe.