sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Coral)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (shows hexagonal pattern when wet), Luster: Dull to vitreous, Structure: Hexagonal colonial coral cells, Cleavage: None

Identified May 21, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (shows hexagonal pattern when wet), Luster: Dull to vitreous, Structure: Hexagonal colonial coral cells, Cleavage: None

Formation & geological history

Formed through the fossilization of colonial rugose coral during the Devonian Period (approx. 350 million years ago) in warm, shallow seas. Glaciations later plucked these stones from the bedrock and spread them across Michigan.

Uses & applications

Highly valued as a state stone, used for biological study, lapidary arts (jewelry, cabochons, carvings), and as a popular collector's item.

Geological facts

It is the state stone of Michigan. The name 'Petoskey' comes from an Ottawa Indian Chief, Pet-O-Sega. The stone's patterns become clearly visible only when the stone is wet or polished.

Field identification & locations

Identify in the field by looking for 'six-sided cells' or sunburst patterns on gray stones, especially on the shores of Lake Michigan. Best found after a storm or in the spring when the ice thaws.