sedimentary

Petoskey Stone

Hexagonaria percarinata

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry, reveals hexagonal patterns when wet. Luster: Dull (natural) to Vitreous (polished). Structure: Hexagonal corallites. Cleavage: None. Specific Gravity: 2.6-2.7.

Color: Gray to tan when dry, reveals hexagonal patterns when wet
Luster: Dull (natural) to Vitreous (polished)

Identified May 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period about 350 million years ago. It is a fossilized rugose coral that lived in warm, shallow saltwater seas covering Michigan. Glacial action eventually spread the fossils across the Great Lakes region.

Uses & applications

Commonly used in jewelry, decorative items, as a popular state souvenir, and highly prized by rock collectors.

Geological facts

It is the official state stone of Michigan. The name comes from Ottawa Chief Pet-O-Sega, and the patterned 'eyes' are actually individual coral polyps.

Field identification & locations

Identify in the field by looking for the distinctive hexagonal pattern on gray cobbles along shorelines. Pattern is best seen when the stone is wet. Primarily found on the shores of Lake Michigan and Lake Huron.