sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3.5 (Calcite) to 7 (Silicate), Color: Gray to light brown, Luster: Dull to vitreous, Crystal Structure: Hexagonal honeycombed pattern of corallites, Cleavage: None (fossilized structure).
- Hardness
- 3
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Physical properties
Hardness: 3.5 (Calcite) to 7 (Silicate), Color: Gray to light brown, Luster: Dull to vitreous, Crystal Structure: Hexagonal honeycombed pattern of corallites, Cleavage: None (fossilized structure).
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) from colonial rugose coral in warm, shallow seas. As the seas dried, the skeletons were buried in sediment and eventually fossilized with calcite and silica.
Uses & applications
Primarily used for jewelry, lapidary art, and collection. It is the official state stone of Michigan.
Geological facts
The name 'Petoskey' comes from the Ottawa Chief Petosegay. The unique pattern is only visible when the stone is wet or polished; dry specimens often look like ordinary gray limestone.
Field identification & locations
Identify by wetting the surface to reveal the 'sunburst' or hexagonal pattern. Commonly found along the shores of Lake Michigan and Lake Huron, especially after storms or spring ice melts.
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