sedimentary

Fossilized Rugose Coral (Petoskey Stone type)

Hexagonaria percarinata (Order: Rugosa)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs); Color: Grey, tan, or white with distinct honeycomb/star patterns; Luster: Dull to pearly when raw, vitreous when polished; Structure: Calcite fossil replacements.

Hardness: 3-4 (Mohs)
Color: Grey, tan, or white with distinct honeycomb/star patterns
Luster: Dull to pearly when raw, vitreous when polished

Identified June 20, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs); Color: Grey, tan, or white with distinct honeycomb/star patterns; Luster: Dull to pearly when raw, vitreous when polished; Structure: Calcite fossil replacements.

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) from the fossilized remains of ancient colonial coral reefs in warm, shallow saltwater seas.

Uses & applications

Used primarily for lapidary work, jewelry (pendants and beads), ornamental stones, and as a popular souvenir or collector's item in the Great Lakes region.

Geological facts

The Petoskey stone is the state stone of Michigan. The pattern seen on the surface results from the skeleton of the coral polyps, which contained living tissue during the Devonian age.

Field identification & locations

Look for the distinctive 'six-sided' or 'star' patterns on smooth, water-worn grey pebbles along shorelines. Wetting the stone often reveals the pattern hidden by the dry surface.