Petoskey stones are unusual gray stones with spots that are the result of ice sheets, bedrock & coral colliding 350 million years ago and fossilizing. Sort of.
Anyway, they can only be found in the northwestern tip of Michigan's lower peninsula, which makes them a local treasure. Spend more than 15 minutes on the beach and someone will propose a "hunt" for petoskey stones. Shown here are my nieces, combing the beach for petoskey stones and looking a lot like the evolution of early man. Welcome to the Stone Age, baby.