fossil

Petoskey Stone

Hexagonaria percarinata (Calcite / CaCO3)

Hardness: 3 (Mohs scale). Color: Grey, tan, or light brown, revealing a repeating hexagonal cell pattern when wet or polished. Luster: Vitreous to dull. Cleavage: Perfect rhombohedral in calcite areas. Specific Gravity: ~2.71.

Hardness: 3 (Mohs scale)
Luster: Vitreous to dull

Identified July 21, 2026More fossil →

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Physical properties

Formation & geological history

Formed through the fossilization of ancient rugose coral reefs during the Devonian Period (about 350 million years ago) when Michigan was covered by a warm, shallow sea. Organic material was gradually replaced by calcite mineral deposits.

Uses & applications

Popular for lapidary arts, stone carving, jewelry (cabochons, pendants), decorative display pieces, souvenirs, and geological collections.

Geological facts

Petoskey Stone was designated as the official state stone of Michigan in 1965. The name comes from Ottawa Chief Pet-O-Sega, meaning 'rays of rising sun', which matches the distinctive sunburst/hexagonal structure of the fossilized coral corallites.

Field identification & locations

Easily identified by the distinctive hexagonal cellular pattern visible when the stone is wet or polished. Most famously found along the beaches of Lake Michigan and Lake Huron in northern Lower Peninsula Michigan, especially near Petoskey.

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