sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs), Color: Gray/brown when dry; patterned when wet or polished, Luster: Vitreous to dull, Structure: Hexagonal colonial cells with radiating septa, Specific Gravity: ~2.7

Hardness: 3-4 (Mohs), Color: Gray/brown when dry
Color: Gray/brown when dry

Identified June 11, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs), Color: Gray/brown when dry; patterned when wet or polished, Luster: Vitreous to dull, Structure: Hexagonal colonial cells with radiating septa, Specific Gravity: ~2.7

Formation & geological history

Formed during the Devonian Period about 350 million years ago. It is fossilized coral (Hexagonaria) that was altered as silica-rich groundwater replaced the calcium carbonate structure during the rock-forming process.

Uses & applications

Used primarily as decorative carvings, jewelry, lapidary art, and souvenirs. It is highly valued as a collectible and is the official state stone of Michigan.

Geological facts

A Petoskey stone is actually a fossil of a colonial coral that lived in warm, shallow seas. When dry, it looks like ordinary limestone, but when wet or polished, the distinctive 'six-sided' star pattern of the coral skeletons appears.

Field identification & locations

Identify in the field by looking for a 'honeycomb' or star-like pattern on the surface of gray stones, especially along shorelines of Lake Michigan and Lake Huron. It often requires being wet to see the pattern clearly.