The Petoskey Stone: A Coral Reef in Your Pocket
Near Petoskey, Michigan
If you walk this shoreline with your eyes down, you are hunting for a creature that lived and died long before the dinosaurs. The Petoskey stone is not really a stone at all. It is fossilized coral, the skeleton of a colonial animal called Hexagonaria, which built vast reefs in a warm, shallow, tropical sea roughly three hundred fifty million years ago, during the Devonian period.
Back then the land that is now Michigan sat near the equator, drowned beneath clear salt water that swarmed with brachiopods, sea lilies, trilobites, and armor plated fish. Each little six sided cell you can see was once the home of a single coral polyp, with a mouth at the center and tentacles reaching out to catch food. When the animal died, sediment filled the skeleton and slowly turned it to stone.
For hundreds of millions of years these fossil reefs lay buried in the bedrock. Then, during the last ice age, glaciers a mile thick ground across the peninsula, plucked the fossilized coral loose, rounded off its edges, and scattered it across the northwestern shore of the Lower Peninsula. That is why you find Petoskey stones here, tumbled smooth and mixed in with ordinary gravel, especially along the beaches of Little Traverse Bay.
They are famously shy. When it is dry, a Petoskey stone looks like a dull gray rock, and most people walk right past it. Wet it, or hold it in the shallows, and the honeycomb pattern of the ancient coral suddenly blooms across the surface.
Rock hunters learn to search the water line just after the waves pull back, and to slow down when they find one, because the same glaciers that spread these stones tended to drop them in clusters. The fossil became a point of deep local pride, and in nineteen sixty-five the Michigan legislature named it the official state stone. It even carries the name of the Odawa leader for whom this town is named, drawn from the land where the fossil was found in abundance.
So when you slip one into your pocket here, you are holding a piece of a coral reef, a glacier, and a people's history all at once.
