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

Fossilized Coral (Petoskey Stone / Favosites)

Hexagonaria percarinata / Favosites sp.

Hardness: 3-4 on Mohs scale. Color: Gray, tan, brown, or white. Luster: Dull to earthy (silky to vitreous when polished). Structure: Distinctive hexagonal or honeycomb corallite pattern visible on surface. Cleavage: None (calcite cleavage within matrix). Specific gravity: ~2.7.

Hardness
3-4 on Mohs scale
Color
Gray, tan, brown, or white
Luster
Dull to earthy (silky to vitreous when polished)
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Physical properties

Hardness: 3-4 on Mohs scale. Color: Gray, tan, brown, or white. Luster: Dull to earthy (silky to vitreous when polished). Structure: Distinctive hexagonal or honeycomb corallite pattern visible on surface. Cleavage: None (calcite cleavage within matrix). Specific gravity: ~2.7.

Formation & geological history

Formed from ancient marine coral reefs during the Devonian Period (around 350-400 million years ago). As the marine organisms died, their calcium carbonate skeletons were gradually replaced and surrounded by fine sediment, fossilizing over geological time.

Uses & applications

Widely used for lapidary arts, decorative carvings, jewelry (pendants, cabochons), bookends, and popular among fossil and mineral collectors. Michigan's state stone is the Petoskey stone, a type of fossilized Hexagonaria coral.

Geological facts

The individual chambers seen in the pattern are called corallites, which housed living coral polyps millions of years ago. The distinct geometric patterns become strikingly vivid when wet or polished.

Field identification & locations

Look for repeating hexagonal, starburst, or honeycomb cell structures on gray-brown limestone or rock matrix. Commonly found in coastal areas, riverbeds, and gravel pits across Michigan, Ohio, Iowa, and ancient marine basin deposits worldwide.

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