No industrial process is perfect (see: amount of insect material allowed in human food by law). Especially one designed to produce 50 lb bags of rocks for a $20 price point.
This tiny shell probably slipped in with the truckload of oyster shell and was too small to get crushed by the machinery.
This tiny shell probably slipped in with the truckload of oyster shell and was too small to get crushed by the machinery.