sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3 – 3.5 on Mohs scale; Color: Gray to brown when dry, displays honeycomb pattern when wet; Luster: Dull to vitreous; Crystal Structure: Hexagonal corallites; Cleavage: None; Specific Gravity: 2.7.

Hardness: 3 – 3
Color: Gray to brown when dry, displays honeycomb pattern when wet
Luster: Dull to vitreous

Identified May 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the fossilization of ancient colonial coral that lived in warm, shallow Devonian seas approximately 350 million years ago. Glaciation later plucked the stones from the bedrock and deposited them across the Great Lakes region.

Uses & applications

Primarily used as a state stone for Michigan, in jewelry (pendants, rings), decorative ornaments, souvenirs, and as a popular collector's item.

Geological facts

It is the official state stone of Michigan. The name 'Petoskey' comes from an Ottawa Chief named Pet-O-Sega, meaning 'rising sun' or 'rays of hope.' The pattern is only clearly visible when the stone is wet or polished.

Field identification & locations

Identify in the field by looking for a unique six-sided honeycomb or 'star' pattern on limestone rocks near shorelines. Commonly found on the beaches of Lake Michigan and Lake Huron, especially in the northern lower peninsula of Michigan.

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