Rock Identifier
Fossilized Coral (Petoskey Stone / Favosites) (Hexagonaria percarinata / Favosites sp. (Calcite, CaCO3)) — fossil
fossil

Fossilized Coral (Petoskey Stone / Favosites)

Hexagonaria percarinata / Favosites sp. (Calcite, CaCO3)

AI-generated identification · may be incorrect

Hardness: 3.0-3.5 on Mohs scale. Color: Light gray to tan with darker gray/brown distinct cell structures (corallites). Luster: Dull to vitreous when polished. Structure: Calcareous skeletal fossil matrix.

Hardness
3
Luster
Dull to vitreous when polished
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Physical properties

Hardness: 3.0-3.5 on Mohs scale. Color: Light gray to tan with darker gray/brown distinct cell structures (corallites). Luster: Dull to vitreous when polished. Structure: Calcareous skeletal fossil matrix.

Formation & geological history

Formed during the Devonian Period (around 350-400 million years ago) when warm, shallow seas covered North America. Living coral colonies were buried by sediment, with calcium carbonate replacing organic tissues over millions of years.

Uses & applications

Popular among lapidaries and fossil collectors. Often polished to make cabochons, jewelry, paperweights, and decorative ornaments.

Geological facts

These beach-worn fossil corals are famous along the shores of Lake Michigan and the Great Lakes region. When wet or polished, the distinct pattern of individual coral polyps becomes vividly visible.

Field identification & locations

Look for rounded, water-smoothed pebbles displaying a distinct honeycomb or star-like pore pattern (corallites). Commonly found along freshwater lake beaches, gravel pits, and limestone outcrops. Wetting the stone reveals hidden patterns.

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