Rock Identifier
Fossil Coral (Petoskey Stone / Hexagonaria) (Hexagonaria percarinata (Silicified/Calcite Replacement Coral)) — fossil
fossil

Fossil Coral (Petoskey Stone / Hexagonaria)

Hexagonaria percarinata (Silicified/Calcite Replacement Coral)

Hardness: 3-4 (calcite) to 7 (if quartz replaced/silicified). Color: Grey, beige, brown, or tan. Luster: Dull to vitreous inside chambers. Structure: Hexagonal pattern of corallite chambers with central dark eyes and radial septa.

Hardness
3-4 (calcite) to 7 (if quartz replaced/silicified)
Color
Grey, beige, brown, or tan
Luster
Dull to vitreous inside chambers
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Physical properties

Hardness: 3-4 (calcite) to 7 (if quartz replaced/silicified). Color: Grey, beige, brown, or tan. Luster: Dull to vitreous inside chambers. Structure: Hexagonal pattern of corallite chambers with central dark eyes and radial septa.

Formation & geological history

Formed during the Middle Devonian period (about 350 million years ago) from ancient rugose coral reefs in warm, shallow saltwater seas. Over time, the living tissues died and were buried under sediments, undergoing mineralization.

Uses & applications

Popular among rockhounds for lapidary work, jewelry, paperweights, display specimens, and educational collections.

Geological facts

Petoskey stones are Michigan's official state stone. When dry, the rock often looks like ordinary grey limestone, but when wet or polished, the intricate honeycomb honeycomb-like coral structure vividly emerges.

Field identification & locations

Look for honeycomb-like hexagonal repeating patterns on water-worn, rounded pebbles along lake shores and gravel pits, particularly around the Great Lakes region.

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