The Petoskey Stone is a highly sought-after fossil recognized for its distinctive, intricate pattern and its status as the official state stone of Michigan. Collectors and enthusiasts are drawn to this unique specimen, which often resembles a honeycomb or a turtle shell. Found primarily in the Great Lakes region, the stone holds a rich history spanning hundreds of millions of years, tracing back to a time when Michigan was covered by an ancient sea. Its appearance offers a tangible link to the geological past of the North American continent.
Defining the Petoskey Stone
A Petoskey Stone is not a rock in the traditional sense, but rather a fossilized colonial coral, specifically identified as Hexagonaria percarinata. The striking pattern is a cross-section of the individual skeletal structures, known as corallites, which once housed the living coral polyps. These corallites are tightly packed, forming the characteristic six-sided, or hexagonal, shapes visible on the stone’s surface.
The center of each hexagon marks the location of the former mouth of the coral polyp, often appearing as a dark spot from which thin lines radiate outward. These radiating lines are the preserved vertical skeletal partitions, called septa. The fossil’s composition is primarily calcium carbonate, or calcite, the mineral that replaced the original organic material during preservation. This fossil is a type of rugose coral, an extinct group that thrived during the Paleozoic Era.
Geological History and Formation
The Petoskey Stone originated approximately 350 to 419 million years ago, during the Devonian Period. At this time, the landmass that would become Michigan was situated closer to the equator and submerged beneath a warm, shallow, saltwater sea. These conditions were perfect for the flourishing of massive coral reefs, including the colonies of Hexagonaria percarinata.
When the coral colonies died, their calcium carbonate skeletons were buried by layers of marine sediment. Over millions of years, fossilization occurred as the original organic material was gradually replaced by minerals like calcite. This mineral replacement preserved the coral’s intricate internal structure, turning the fragile skeleton into solid limestone rock. The fossil-bearing rock strata, part of the Traverse Group, remained buried until later geological events brought them to the surface.
The final step involved the Pleistocene Ice Age, which began about two million years ago. Glacial ice sheets scoured the bedrock, plucking fragments of the fossilized coral and scattering them across the landscape. This glacial action broke the larger coral heads into smaller pieces and, through grinding and tumbling, smoothed their rough edges. The resulting pebbles were deposited in the glacial till, explaining why the stones are now found along beaches and in inland gravel deposits.
Identification and Preparation
When dry and unpolished, a Petoskey Stone often resembles an ordinary piece of dull gray or brown limestone. The six-sided pattern, however, becomes instantly apparent and highly contrasting once the surface is wet. This visual change is the single most reliable way for collectors to identify the stone in its natural environment along the shores of the Great Lakes.
The best places to search are the beaches along the northern half of Michigan’s Lower Peninsula, particularly near the towns of Petoskey and Charlevoix. Enthusiasts often look for stones in shallow water or after a rain shower, as moisture enhances the pattern’s visibility. To bring out the full luster and intricate detail, the stone requires preparation.
To achieve the glossy finish seen in jewelry and souvenirs, the stone must be polished. Hand-polishing involves using a series of increasingly finer-grit wet sandpaper, starting with a coarse grit like 80 and progressing to very fine grits such as 600. This sanding removes surface imperfections and prepares the calcite for the final step: buffing it with a polishing compound to achieve a mirror-like shine. Due to the stone’s relative softness, it is advised to avoid placing them in a hobby rock tumbler, which can damage the fossil structure.
Significance and Geographical Context
The Petoskey Stone was officially designated as the State Stone of Michigan in 1965, cementing its status as a symbol of the state’s natural heritage. The stone and the city of Petoskey share a name with a revered figure from Native American history. The name is an anglicized version of Pet-O-Sega, an Ottawa Chief whose name means “rays of the rising sun.”
Local legend suggests that the corallite pattern, with its radiating lines, visually connects to the meaning of the Chief’s name. The town, established in 1873, was named in his honor, and the stones found nearby became popular souvenirs for Victorian-era tourists. Today, collecting these unique fossils remains a significant part of the regional identity and tourism draw.