fossil

Petoskey Stone (Fossilized Rugose Coral)

Hexagonaria percarinata

Hardness 3-4 on Mohs scale, grey to brownish-grey color, dull to waxy luster, composed primarily of calcite (CaCO3). Displays a characteristic hexagonal pattern of fossilized coral polyp chambers when wet or polished.

Identified July 27, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) when Michigan was covered by a warm, shallow sea. Ancient rugose coral colonies were buried, petrified, and transformed into calcite over millions of years, then eroded by glaciers.

Uses & applications

Michigan's official state stone, highly popular for lapidary work, jewelry making, polished specimens, paperweights, and souvenirs.

Geological facts

Named after Chief Pet-O-Sega, an Ottawa Indian chief whose name means 'rays of rising sun', referencing the sunburst pattern seen in the hexagonal coral cells.

Field identification & locations

When dry, it appears as a plain grey rock; wetting or polishing reveals the distinctive hexagonal coral structures. Commonly found along the shores of Lake Michigan and Lake Huron, especially around Petoskey, Michigan.

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