Pearl Found in Grilled Oysters

My brother and I picked up 8 live oysters at the grocery store as an appetizer for our most recent BBQ. We ate 4 raw and put the last 4 on the grill. He bit into one of the ones that came off of the grill and thought that he had gotten a piece of shell or sand, but after looking closer realized it was a small pearl, partially covered in a black crust type of stuff. The pearl measures between 1/16th and 1/8th of an inch across. It was a cool find....