Rock Identifier
Coral Fossil (Petoskey Stone or Rugose Coral) (Hexagonaria percarinata (Rugose coral)) — fossil
fossil

Coral Fossil (Petoskey Stone or Rugose Coral)

Hexagonaria percarinata (Rugose coral)

AI-generated identification · may be incorrect

Mohs hardness of 3-4, colors ranging from gray to brown, vitreous to dull luster, hexagonal corallite structures, and distinct mottled patterns when wet.

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Physical properties

Mohs hardness of 3-4, colors ranging from gray to brown, vitreous to dull luster, hexagonal corallite structures, and distinct mottled patterns when wet.

Formation & geological history

Formed during the Devonian period about 350 million years ago when ancient coral reefs accumulated and were later fossilized in sedimentary limestone.

Uses & applications

Used primarily as a decorative lapidary stone, in polished cabochons for jewelry, and as a popular souvenir or collector specimen.

Geological facts

It is the official state stone of Michigan and features distinctive honeycomb-like patterns that become much more prominent when wet or polished.

Field identification & locations

Easily identified by its pattern of six-sided coral cells exposed on weathered rock surfaces; commonly found in gravel pits and glacial drift.

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Bioclastic limestone, composed chiefly of calcite (CaCO₃)

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sedimentary
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Fossil oyster shell

Oyster (family Ostreidae); shell composed chiefly of calcite (CaCO₃)

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