It's a nacreous concretion formed around a foreign body, like a grain of sand
a grain of sand) composed of filmy layers of... [DOC] [TLE] '1"' lI - Fishery Bulletin [PAR] shell of some foreign body, as a grain of sand, an e~g of the mollusk itself, or 
perhaps .... resultingiu the formation of pearls or pearly concretions.