It's a nacreous concretion formed around a foreign body, like a grain of sand
If a foreign body, such as one of its own eggs, a grain of 
sand or a minute... [DOC] [TLE] Concretion - Wikipedia [PAR] A concretion is a hard, compact mass of matter formed by the precipitation of 
mineral cement ... Concretions are formed from mineral precipitation around 
some kind of nucleus while a nodule is a replacement body.