It's a nacreous concretion formed around a foreign body, like a grain of sand
... body of varied but usually roundish shape that is an abnormal 
nacreous ... around a grain of sand, a parasite, or some other foreign object: it is 
used as a... [DOC] [TLE] Concretions - Paleontological Research Institution [PAR] Concretions form as minerals within a rock segregate and begin to ...