fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3-3.5 (Mohs scale), Color: light grey to cream when dry, dark patterns when wet, Luster: dull to earthy (unpolished) or vitreous (polished), Crystal Structure: Orthorhombic/Hexagonal patterns

Hardness: 3-3

Identified July 7, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) from fossilized rugose coral colonies in shallow tropical seas.

Uses & applications

Commonly used in jewelry, as decorative souvenirs, paperweights, and highly valued by fossil collectors.

Geological facts

It is the state stone of Michigan. The unique 'eye' pattern only becomes clearly visible when the stone is wet or polished because the calcium carbonate structures have different porosities.

Field identification & locations

Identified by the distinct six-sided (hexagonal) coral patterns. Found primarily on the shores of Lake Michigan and Lake Huron. Collectors look for them on beaches after rain or among wave-washed pebbles.