fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3-4 (Mohs), Color: Grey to brown (displays hexagonal pattern when wet or polished), Luster: Dull to pearly, Crystal structure: Hexagonal (coral colony), Cleavage: None

Identified July 19, 2026More fossil →

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Physical properties

Hardness: 3-4 (Mohs), Color: Grey to brown (displays hexagonal pattern when wet or polished), Luster: Dull to pearly, Crystal structure: Hexagonal (coral colony), Cleavage: None

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) from fossilized rugose coral colonies in shallow tropical seas. Glacial activity later distributed them across the Michigan basin.

Uses & applications

Used in jewelry, decorative carvings, aquarium decor, and as a popular souvenir or collector's item in the Great Lakes region.

Geological facts

The Petoskey Stone is the official state stone of Michigan. The pattern consists of fossilized coral polyps, and the name comes from an Ottawa Chief, Pet-O-Sega.

Field identification & locations

Identified by 'eye' patterns (hexagonal cells). In the field, look for them on beaches; they are often difficult to see when dry but the pattern 'pops' when they are wet. Common in the Traverse Group geological formation.