fossil

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3.0-3.5 (Calcite); Color: Gray to tan when dry, reveals hexagonal patterns when wet or polished; Luster: Dull to vitreous; Crystal Structure: Trigonal (as calcite replacement); Cleavage: Perfect rhombohedral.

Hardness: 3
Luster: Dull to vitreous

Identified July 13, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) in warm, shallow saltwater seas. The living coral colonies were later covered by sediment and replaced by calcite during fossilization.

Uses & applications

Highly valued as Michigan's state stone, used in jewelry (pendants, rings), decorative ornaments, bookends, and as a popular collectible for rockhounds.

Geological facts

The name 'Petoskey' comes from the Ottawa Chief Pet-O-Sega. The stone is unique because the distinct 'sunburst' or hexagonal pattern is actually the skeleton of individual coral polyps.

Field identification & locations

Identified by the unique six-sided star/hexagon patterns (corallites). Primarily found along the shores of Lake Michigan and Lake Huron. Most visible when wet or polished.

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