sedimentary
Petoskey Stone (Fossilized Coral)
Hexagonaria percarinata
Hardness: 3-4 (Mohs), Color: gray to tan, Luster: dull to vitreous when polished, Crystal structure: hexagonal patterns of calcite, Specific gravity: approx 2.7
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Physical properties
Hardness: 3-4 (Mohs), Color: gray to tan, Luster: dull to vitreous when polished, Crystal structure: hexagonal patterns of calcite, Specific gravity: approx 2.7
Formation & geological history
Formed from fossilized colonial rugose coral living during the Devonian Period (approx. 350 million years ago) in shallow seas, later compressed into limestone and transported by glaciers.
Uses & applications
Commonly used for jewelry (pendants, rings), decorative ornaments, collection, and as the official state stone of Michigan.
Geological facts
The unique 'eye' pattern is the fossilized skeleton of individual coral polyps. The name 'Petoskey' comes from an Ottawa Indian Chief named Petosegay meaning 'beams of rising sun'.
Field identification & locations
Identified by the distinct 6-sided hexagonal patterns visible when wet or polished. Commonly found on the shores of Lake Michigan and Lake Huron. Collectors should look for the honeycomb pattern.
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