These amazing stones are Petoskey stones; composed of fossilized skeletons of Hexagonaria percarinata, a type of coral from coral reefs that once covered all of what is now the state of Michigan, USA, during the ancient Devonian period, some 350 million years ago. The stones were formed as a result of glaciation, in which sheets of ice scrapped the bedrock, picking up fragments, and then grinding off their rough edges and depositing them in the northwestern portion of Michigan’s lower peninsula. You could say Petoskey stones are actual pieces of coral reef, and when dry the stone resembles ordinary limestone but when wet or polished, the distinctive mottled pattern of the six-sided coral fossils emerges. This prehistoric fossil is found across the state of Michigan along lakeshores and rivers in sediments commonly called the Traverse group. Since 1965, the Petoskey stone is Michigan’s official state stone.