sedimentary

Petoskey Stone (Fossilized Coral)

Hexagonaria percarinata

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs), Color: gray to tan when dry, mottled patterns when wet; Luster: dull to vitreous when polished; Structure: colonial rugose coral with hexagonal cells.

Color: gray to tan when dry, mottled patterns when wet
Luster: dull to vitreous when polished

Identified June 20, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs), Color: gray to tan when dry, mottled patterns when wet; Luster: dull to vitreous when polished; Structure: colonial rugose coral with hexagonal cells.

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) in warm shallow seas as coral colonies; later fossilized and glaciated during the Pleistocene.

Uses & applications

Commonly used as a decorative stone, lapidary projects, souvenirs, and jewelry when polished.

Geological facts

It is the state stone of Michigan. The unique 'eye' pattern is only visible once the stone is wet or polished, which highlights the individual coral polyps.

Field identification & locations

Look for rounded grayish pebbles on the shores of Lake Michigan. The distinct hexagonal pattern (resembling a honeycomb or rays of 'Petoskey' sun) is the primary identifier.