fossil

Petoskey Stone (Fossilized Rugose Coral)

Hexagonaria percarinata

AI-generated identification · may be incorrect

Hardness: 3-3.5 on Mohs scale; Color: Gray, tan, or brown; Luster: Dull (natural) to Vitreous (polished); Structure: Hexagonal colonial coral cells with radiating lines (septa); Composition: Calcite

Hardness: 3-3
Color: Gray, tan, or brown
Luster: Dull (natural) to Vitreous (polished)

Identified July 20, 2026More fossil →

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Physical properties

Hardness: 3-3.5 on Mohs scale; Color: Gray, tan, or brown; Luster: Dull (natural) to Vitreous (polished); Structure: Hexagonal colonial coral cells with radiating lines (septa); Composition: Calcite

Formation & geological history

Formed during the Devonian Period about 350 million years ago. These are fossilized colonial corals that lived in warm, shallow seas covering what is now the Michigan Basin. Glacial action eventually moved and distributed the fossils.

Uses & applications

Highly prized as a state stone for Michigan, used in lapidary work for jewelry, pendants, paperweights, and as collector specimens.

Geological facts

The name comes from Ottawa Chief Pet-O-Sega. While they appear as ordinary gray stones when dry, their unique hexagonal pattern 'blooms' and becomes strikingly visible when wet or polished.

Field identification & locations

Can be found along the shores of Lake Michigan and Lake Huron. Identify by the hexagonal honeycomb pattern. It is the official state stone of Michigan. Collectors often look for them after a storm when waves have turned up the shoreline.