It's a nacreous concretion formed around a foreign body, like a grain of sand
a grain of sand), composed of filmy layers of carbonate of lime interstratified 
with... [DOC] [TLE] Pearl dictionary definition | pearl defined - YourDictionary [PAR] pearl definition: A pearl is defined as a smooth round gem formed in the shell of 
an ... shell of the oyster, cut a slit in its mantle to insert an irritant such as a grain 
of sand.