Is this a Petoskey stone?
A fossil colonial coral whose surface often shows repeating cells that look like honeycomb, flowers, or small suns.
Three quick checks
Start with what you can see
Look for repeating shapes across the surface, add a little water to reveal a faint pattern, and check more than one face of the stone.
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Look for repetition
The shapes should repeat across the surface rather than appear as one isolated mark.
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Add a little water
Wetness can make a faint pattern easier to see.
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Turn it over
Check more than one face. Weathering can hide the pattern on a dry surface.
One coral colony, many repeating cells
Each round or six-sided shape is a cross-section of one corallite, the small skeletal chamber that housed an individual coral animal within the colony. Fine lines radiate from the center and create the familiar sunburst look.
Look for the whole pattern Other fossil corals occur in Michigan, so one circular mark alone is not enough. Compare the pattern across more than one area of the stone.
A fossil from Michigan's ancient sea
- What it is
- Fossil colonial coral
- Scientific name
- Hexagonaria percarinata
- Approximate age
- About 350 million years
- Michigan symbol
- Michigan state stone since 1965
What would you like to do with it?
Take the full field guide to the beach
The Trading Post's field guide includes clear photos, a broader visual identification guide, tools, tips, and practical help for rockhounding around Lake Michigan.
