“Petoskey Stones” are now the official stone of Michigan. It’s a fossil coral of the genus hexagonaria. This coral lived approximately 350 million years ago in the sea which used to cover Michigan. It is a compound coral in that many animals lived together in a colony. In each small hexagon chamber is the prehistoric remains of a salt water marine life. It is called “Petoskey Stone” because it is found mostly around Petoskey in the area of Little Traverse Bay.