fossil
Petoskey Stone (Weathered/Rough Form)
Hexagonaria percarinata
Hardness 3-4 (Mohs), Gray to tan base, Dull to waxy luster when dry, Hexagonal coral pattern (corallites) with a central pit (calyx), Specific gravity 2.7.
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Physical properties
Hardness 3-4 (Mohs), Gray to tan base, Dull to waxy luster when dry, Hexagonal coral pattern (corallites) with a central pit (calyx), Specific gravity 2.7.
Formation & geological history
Fossilized colonial rugose coral formed during the Devonian period (approx. 350 million years ago) in a shallow, warm sea that once covered Michigan.
Uses & applications
State stone of Michigan, used in souvenir jewelry, polished specimens for collecting, and lapidary art.
Geological facts
The name comes from Chief Pet-O-Sega, meaning 'rising sun'. The stones are hexagonal skeletons of extinct coral. When dry, they look like ordinary limestone; the pattern only reveals itself clearly when wet or polished.
Field identification & locations
Commonly found on the shores of Lake Michigan and Lake Huron. Look for the 'honeycomb' or 'spiderweb' pattern on smooth, water-worn gray stones near the shoreline, especially after a storm.
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