fossil

Fossil Coral

Hexagonaria or Rugose Coral (petoskey stone / fossilized rugose or tabulate coral)

Hardness of 3 to 4 on the Mohs scale (for the calcite/aragonite replacement, though often silicified to quartz hardness 7); colors include grey, white, tan, and brown; vitreous to dull luster; hexagonal or tubular corallite structures; specific gravity around 2.6 to 2.7.

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed in warm, shallow ancient seas (often during the Devonian period, roughly 350 to 400 million years ago) when coral reefs grew, eventually becoming buried in sediment and replaced by minerals through permineralization or silicification.

Uses & applications

Used primarily in lapidary arts, making polished stones, jewelry, decorative ornaments, and as educational or collector specimens.

Geological facts

Fossil corals are the state stone of Michigan (specifically the Petoskey stone variety). The distinct pattern shows the individual coral polyps (corallites) that lived together in colonies hundreds of millions of years ago.

Field identification & locations

Easily identified by the honeycomb or star-like patterns of the corallites visible on the weathered surface. Commonly found in limestone-rich sedimentary regions, ancient seabed exposures, and glacial drift.

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Crinoid Stem Fossil in Matrix

Phylum Echinodermata, Class Crinoidea

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Sedimentary Outcrop (Sandstone/Shale)

Sedimentary bedrock formation (variable composition)

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Chert with Fossil Brachiopod

Microcrystalline Quartz (SiO2) with Calcitic Brachiopod inclusion

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Fossilized Crinoid Stem in Limestone

Crinoidea (Class) in Calcium Carbonate (CaCO3) matrix

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Fossiliferous Limestone

Limestone with embedded fossil cross-section

sedimentary

Fossiliferous Limestone with Crinoid stems

Limestone (CaCO3) containing Crinoidea fossils

sedimentary