Rock Identifier
Fossilized Coral (Petoskey Stone / Hexagonaria) (Hexagonaria percarinata / Rugosa coral) — fossil
fossil

Fossilized Coral (Petoskey Stone / Hexagonaria)

Hexagonaria percarinata / Rugosa coral

AI-generated identification · may be incorrect

Hardness: 3-4 on Mohs scale (calcite-based). Color: Gray, beige, light brown, white. Luster: Dull to vitreous (when polished). Structure: Distinctive hexagonal/honeycomb corallite patterns visible on the surface.

Hardness
3-4 on Mohs scale (calcite-based)
Color
Gray, beige, light brown, white
Luster
Dull to vitreous (when polished)
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Physical properties

Hardness: 3-4 on Mohs scale (calcite-based). Color: Gray, beige, light brown, white. Luster: Dull to vitreous (when polished). Structure: Distinctive hexagonal/honeycomb corallite patterns visible on the surface.

Formation & geological history

Formed during the Middle Devonian period (around 350-400 million years ago) when ancient warm, shallow seas covered North America. Dead coral colonies were buried under sediment and gradually replaced by calcite and silica.

Uses & applications

Used in jewelry, decorative carvings, paperweights, lapidary art, and popular among fossil collectors.

Geological facts

Petoskey stones are Michigan's official state stone. The characteristic pattern comes from individual coral polyps whose skeletons (corallites) formed hexagonal chambers.

Field identification & locations

Identified by the six-sided corallite structures with radiating lines (septa) pointing toward a central dark spot. Often easier to see when the stone is wet. Commonly found on beaches along Lake Michigan and Huron.

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