Petoskey stones are fossilized colony corals (Hexagonaria percarinata). Their origin is traced back to Devonian seas that covered Michigan's Lower Peninsula about 350 million years ago. When glaciers scraped the bedrock surface, fragments of this rock were carried and deposited elsewhere, primarily in the northern half of the Lower Peninsula. In 1965, the Michigan legislature became the first in the nation to select a fossil as its state stone.