More about the State 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.