Petoskey stones are distinctive geological finds, often discovered along Michigan’s shores. Many wonder if these unique items are simply unusual rocks or something more. The question of whether a Petoskey stone is a fossil reveals a deep connection to ancient Earth history, offering a tangible link to a prehistoric past.
What is a Petoskey Stone
A Petoskey stone is a pebble-shaped rock, often dull and gray when dry. Its true character emerges when wet or polished, revealing a striking pattern of six-sided shapes. This distinctive appearance makes the stone highly recognizable. Named after Petoskey, Michigan, where they are abundant, this unique stone was designated Michigan’s state stone in 1965. Its presence is strongly associated with the Great Lakes region, especially Lake Michigan’s coastlines.
Their Fossilized Nature
Petoskey stones are fossils, preserving the remains of ancient colonial rugose corals. They are fossilized Hexagonaria percarinata, a species of coral that thrived during the Devonian Period, 350 to 400 million years ago. The hexagonal patterns visible on the stone are fossilized corallites, which were the skeletal cups of individual coral polyps. Each hexagon represents a single organism within a larger coral colony that once inhabited a warm, shallow sea covering what is now Michigan.
How Petoskey Stones Formed
The formation of Petoskey stones began when ancient corals died and their calcium carbonate skeletons accumulated on the seafloor. Over time, these skeletal remains underwent fossilization, primarily through mineral replacement. The original organic material was gradually replaced by minerals, mostly calcite, preserving the hexagonal structure of the corallites within the limestone. Millions of years later, during the last Ice Age, massive glaciers scoured the Michigan landscape, plucking these fossilized coral fragments from the bedrock. The glaciers then transported and deposited these stones across much of Michigan, rounding their edges.
Finding and Identifying Petoskey Stones
Petoskey stones are commonly found along Great Lakes beaches, particularly Lake Michigan, from Traverse City north to the Petoskey and Charlevoix areas. They can also be discovered in inland gravel deposits, carried by glacial activity. To identify a Petoskey stone, look for the unique hexagonal pattern that becomes prominent when the stone is wet. Many enthusiasts carry a water bottle to wet dry stones, which makes the distinctive “honeycomb” or “sunburst” pattern visible. When collecting, be aware of local regulations; Michigan state lands allow collecting up to 25 pounds per year, but national parks prohibit removal.