sedimentary

Petoskey Stone

Hexagonaria percarinata

Hardness: 3.0-3.5 (Mohs), Color: Gray to brown base with white hexagonal patterns, Luster: Silky to matte (dull when dry, pops when wet), Structure: Hexagonal colonial coral skeletons, Cleavage: None

Hardness: 3

Identified June 30, 2026More sedimentary →

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Physical properties

Hardness: 3.0-3.5 (Mohs), Color: Gray to brown base with white hexagonal patterns, Luster: Silky to matte (dull when dry, pops when wet), Structure: Hexagonal colonial coral skeletons, Cleavage: None

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) from colonial rugose coral (Hexagonaria) in ancient warm, shallow seas that once covered Michigan. Glacial action eventually eroded and deposited the stones across the state.

Uses & applications

State stone of Michigan, popular for jewelry, lapidary art, paperweights, and as a highly sought-after fossil specimen for collectors.

Geological facts

The name 'Petoskey' comes from the Ottawa Chief Pet-O-Sega, meaning 'Rising Sun'. Though they look like ordinary gray rocks when dry, their distinctive six-sided coral pattern appears vividly when wet or polished.

Field identification & locations

Identify in the field by wetting the stone to reveal the 'honeycomb' or sunburst patterns. They are primarily found along the shores of Lake Michigan and Lake Huron, especially around the Petoskey and Traverse City areas.