Petoskey stones are distinctive geological formations, recognized by their unique pattern and status as Michigan’s state stone. They are fossilized remnants of ancient coral. When dry, a Petoskey stone may resemble ordinary limestone, but wetting or polishing it reveals a striking hexagonal, honeycomb-like design. This intricate pattern is the preserved structure of the individual coral polyps that once formed a larger colony.
Ancient Marine Life
The biological foundation of a Petoskey stone lies with a specific type of rugose coral known as Hexagonaria percarinata. This colonial coral species thrived during the Devonian Period, approximately 350 to 400 million years ago. During this ancient era, what is now Michigan was situated much closer to the equator, experiencing a warm, shallow, saltwater sea environment. This tropical setting provided ideal conditions for the growth of extensive coral reefs.
Each hexagon visible on a Petoskey stone represents the skeletal remains of a single coral polyp. Each polyp possessed a mouth and tentacles to capture food. The dark “eye” at the center of each hexagon marks where the polyp’s mouth once was, with lines radiating outwards indicating where its tentacles were. As these polyps died, their calcium carbonate skeletons accumulated, eventually forming the dense structures that would become fossilized.
Michigan’s Unique Geological History
The presence of Petoskey stones in Michigan is a direct result of a unique combination of ancient marine conditions and subsequent geological events. During the Devonian Period, the Michigan Basin was a large, subsiding depression covered by a shallow, warm sea. This basin accumulated thick layers of sediment, including carbonate materials from thriving Hexagonaria percarinata coral reefs, forming what is known as the Traverse Group. Over millions of years, these layers of coral and sediment were buried and compressed, leading to the fossilization of the coral into the stone we see today.
Later, during the Pleistocene Epoch, massive continental ice sheets repeatedly covered Michigan. These glaciers acted as powerful geological agents, scouring the bedrock that contained the fossilized coral deposits. As the ice sheets advanced and retreated, they plucked, ground, and transported these stones across the landscape. This glacial action not only eroded the fossil-bearing rock but also polished the fragments, making their distinctive patterns more apparent.
When the glaciers eventually melted, they deposited these polished coral fragments across the northern Lower Peninsula of Michigan. This extensive glacial deposition is why Petoskey stones are found scattered in gravel deposits, along beaches, and in riverbeds throughout the region. This unique geological history, from ancient coral reefs to glacial activity, concentrated Hexagonaria percarinata fossils in this specific area.
Distribution and Uniqueness
While other types of fossilized corals exist globally, the specific combination of Hexagonaria percarinata and the geological processes that made them abundant and easily accessible as polished stones is largely confined to Michigan. While the genus Hexagonaria exists elsewhere, the distinct Hexagonaria percarinata that forms the Petoskey stone is a signature of Michigan. The unique patterns and prevalence make them a special geological and cultural icon for the state.
Petoskey stones are most commonly found along the beaches of Lake Michigan, particularly from Traverse City north to Petoskey. Popular locations include the shorelines around Charlevoix, Northport, and areas within Sleeping Bear Dunes National Lakeshore. The winter ice movement and storm activity along the Great Lakes shores continuously turn over stones, bringing new ones to the surface each spring. While many are found on beaches, they can also be discovered in inland gravel pits and exposed road cuts, further demonstrating the widespread distribution by glacial activity throughout the northern Lower Peninsula.