fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3-4 on Mohs scale. Color: Grey, brownish-grey, or bluish-grey with distinct six-sided coral pattern. Luster: Dull to vitreous when wet or polished. Structure: Massive fossilized coral colonies. Cleavage: None.

Hardness: 3-4 on Mohs scale
Luster: Dull to vitreous when wet or polished

Identified July 24, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Middle Devonian period (around 350 million years ago) when Michigan was covered by a warm, shallow sea. The living rugose coral was buried by sediment and gradually replaced by calcite over millions of years.

Uses & applications

Popular as state stone of Michigan, used widely in lapidary work, jewelry, paperweights, decorative ornaments, and highly sought after by fossil collectors.

Geological facts

Petoskey stone is the official state stone of Michigan. The unique hexagonal pattern represents the individual coral polyps ('corallites') that made up the ancient reef colony. When dry, the rock appears light grey, but when wet or polished, the intricate coral structure becomes vividly visible.

Field identification & locations

Identified by its distinctive honeycomb or 'eye' pattern composed of six-sided cells with light-colored outer walls. Most commonly found along the shores of Lake Michigan and Lake Huron, particularly in the lower peninsula of Michigan around Petoskey.