fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3 to 4 Mohs; color: grey to light brown when dry, patterns reveal when wet; luster: dull to earthy; specific gravity: 2.7; composed of calcite.

Hardness: 3 to 4 Mohs
Color: grey to light brown when dry, patterns reveal when wet
Luster: dull to earthy

Identified July 6, 2026More fossil →

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Physical properties

Hardness: 3 to 4 Mohs; color: grey to light brown when dry, patterns reveal when wet; luster: dull to earthy; specific gravity: 2.7; composed of calcite.

Formation & geological history

Formed during the Devonian Period, approximately 350 to 400 million years ago, as colonial rugose coral in a warm, shallow sea that once covered Michigan.

Uses & applications

Commonly used as a decorative pocket stone, in jewelry (pendants and rings), as paperweights, and for lapidary educational collections.

Geological facts

The Petoskey stone is the state stone of Michigan. The unique 'eye' patterns are actually the individual coral skeletons called corallites.

Field identification & locations

Identified by the distinctive hexagonal pattern. Best found on the shores of Lake Michigan and Lake Huron, especially after a storm or in the spring when the ice thaws.