Yes, there really is a Petoskey Stone!
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.
The soft, living tissue of corals is called the polyp. A limey substance is secreted by the polyp, hardening into corallite -- a skeletal base which supports the polyp and keeps it from being buried alive by bottom debris. Petoskey stones found in Michigan consist of massive corallas of varying sizes. The limey skeletons were replaced by calcite or silica in a cell-by-cell process called petrifaction.
When glaciers scraped the bedrock surface, fragments of this rock were carried and deposited elsewhere, primarily in the north half of the Lower Peninsula. In 1965, the Michigan legislature became the first in the nation to select a fossil as its state stone.
Petoskey stones may be found on beaches, road cuts, ditches, gravel pits and sand blows all over the state. Similar fossils of the Hexagonaria genus occur in many parts of the world, but the "percarinata" is limited to the Traverse Group.