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Specimens of the swallow stone (Chelidonius) include small clasts of agate, gastropod opercula (calcareous lids sealing the apertures of snails), crayfish gastroliths (paired calcareous concretions in the stomach), and possibly fossil fish teeth and larger foraminiferans (a type of unicellular organism). With a long literary pedigree extending from classical to early modern times, swallow stones were believed to have many curative properties utilized in folk medicine, as well as giving protection against evil forces and conferring numerous personal qualities on the owner. The sources presented here are regarded as accessible reflections of contemporary beliefs.
Christopher J. Duffin (Mon,) studied this question.