It's a nacreous concretion formed around a foreign body, like a grain of sand
[DOC] [TLE] Pearls and Pearl Fisheries - JStor [PAR] possesses the same iridescent or nacreous character, and is often known as ... 
Pearls are concretions in the tissues, of the same material as that which 
composes the ... pseudo-pearls may be formed by the mantle over projections 
from the inside of ....