Sedimentary

Petoskey Stone

Hexagonaria percarinata

Hardness: 3 on Mohs scale, Color: Light brown to gray, Luster: Dull to waxy, Crystal Structure: N/A (Fossilized Coral), Cleavage: None, Specific Gravity: Variable depending on mineralization

Identified June 19, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale, Color: Light brown to gray, Luster: Dull to waxy, Crystal Structure: N/A (Fossilized Coral), Cleavage: None, Specific Gravity: Variable depending on mineralization

Formation & geological history

Formed from fossilized rugose coral from the Devonian period, originally deposited in shallow marine environments.

Uses & applications

Used primarily in lapidary arts for cabochons, pendants, and ornamental objects. It is the state stone of Michigan.

Geological facts

Petoskey stones are actually fossilized coral, specifically Hexagonaria percarinata, which lived approximately 350 million years ago. They are distinguished by their hexagonal cell pattern.

Field identification & locations

Identified by its distinctive hexagonal pattern, commonly found along the shores of Lake Michigan and Lake Huron. Collectors often look for them wet, as the pattern becomes more visible.