The Petoskey Stone is Michigan’s state fossil, representing the state’s deep natural history and the ancient life forms that once populated the region. This geological specimen captures millions of years of the state’s past. It is a common find along the shores of the Great Lakes, linking modern-day Michiganders to an ancient, tropical marine environment. The Petoskey Stone was officially designated as Michigan’s state symbol due to its widespread recognition and historical significance within the state.
Identifying Michigan’s State Fossil
The Petoskey Stone is a fossilized colonial coral, scientifically identified as Hexagonaria percarinata. This specimen belongs to the extinct group of rugose corals, which once formed vast reefs in prehistoric oceans. Its name comes from the city of Petoskey, Michigan, where the stones are found in abundance and were historically gathered as souvenirs. The common name is also tied to an Ottawa chief named Pet-O-Sega, whose name translates to “rays of the rising sun.”
The Petoskey Stone is characterized by a distinctive, repeating pattern of six-sided shapes. These shapes are the preserved cross-sections of the coral polyps’ skeletal structures, known as corallites. Each hexagonal unit features a dark center and fine lines radiating outwards, creating a “honeycomb” or “starburst” motif. The stone is composed primarily of calcium carbonate, which replaced the original coral skeleton during fossilization.
When dry, the Petoskey Stone often resembles ordinary gray limestone and can be easily overlooked among shoreline pebbles. Applying water or polishing the surface enhances the contrast, causing the hexagonal pattern to emerge clearly. This visual transformation is why the stones are prized by collectors and lapidary artists for use in jewelry and decorative objects. The fossil represents a single, massive colony where thousands of individual coral organisms lived tightly packed together.
The Petoskey Stone’s Geological Past
The history of the Petoskey Stone stretches back to the Middle Devonian Period, approximately 350 million years ago. During this era, the landmass that would eventually become Michigan was situated near the equator, covered by a warm, shallow, saltwater sea. This tropical environment provided the conditions for the growth of coral reefs, where the Hexagonaria species thrived alongside other marine invertebrates.
These colonial corals built large reef structures, with individual polyps extracting calcium carbonate from the sea to construct their hard skeletons. As the organisms died, their skeletons accumulated on the ocean floor and became buried under layers of marine sediment. The geological process of lithification transformed the ancient coral material into the limestone rock known today as the Petoskey Stone.
The primary source rock for these fossils is the Traverse Group, specifically the Gravel Point Formation, a geological layer found beneath the surface of Michigan’s northern Lower Peninsula. Millions of years after the coral flourished, massive continental ice sheets advanced across the region during the Pleistocene Epoch. These glaciers scraped the fossil-bearing bedrock, plucking out fragments and scattering them across the landscape.
Glacial action smoothed and rounded the sharp edges of the fossil fragments, giving the Petoskey Stones their characteristic pebble shape. The subsequent melting of the ice sheets left large concentrations of these tumbled fossils, particularly along the shores and gravel deposits of Lake Michigan. Today, the movement of ice and water along the coastline continues to expose new specimens each spring, making the beaches a popular destination for rock hunters.
Designation as a State Symbol
The Petoskey Stone’s journey to a state symbol reflects its deep connection to Michigan’s regional identity and local culture. The designation was a legislative act driven by popular support for recognizing this fossil as an official emblem. In 1965, the Petoskey Stone was formally adopted as the state stone of Michigan, cementing its place in the state’s official iconography.
This designation was formalized when Governor George Romney signed Act No. 89 into law on June 28, 1965, following its passage by the state legislature. The stone was a natural choice due to its distinct geological origin and its abundance in the northern parts of the state, particularly around the city that shares its name. It quickly became a significant part of the state’s souvenir and craft industry, frequently polished and set into jewelry.
The fossil’s recognition highlights the importance of the Great Lakes region’s prehistoric past and its enduring link to the present day. For many residents and visitors, finding a Petoskey Stone is a cherished experience, representing a tangible piece of Michigan’s ancient tropical history.