fossil

Petoskey Stone (Fossilized Coral)

Hexagonaria percarinata

Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry, characteristic 'hexagon' patterns appear when wet. Luster: Dull to vitreous when polished. Composition: Calcite replacing ancient coral structures.

Luster: Dull to vitreous when polished

Identified July 7, 2026More fossil →

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Physical properties

Formation & geological history

Formed about 350 million years ago during the Devonian Period when Michigan was a warm, shallow tropical sea. The coral was later covered by sediment and fossilized as the minerals were replaced by calcite.

Uses & applications

State stone of Michigan, used widely in jewelry (pendants, rings), decorative ornaments, collection, and lapidary arts.

Geological facts

The 'eyes' in the stone are ancient colonial coral polyps. The name 'Petoskey' comes from an Ottawa Indian Chief, Pet-O-Sega, meaning 'Rising Sun' or 'Rays of Hope'.

Field identification & locations

Identified by the distinct six-sided coral pattern. Most easily found along the shores of Lake Michigan and Lake Huron, especially after a storm when beaches are churned up. Wetting the stone makes the pattern visible.

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