fossil

Fossil Coral

Hexagonaria / Rugose coral (Fossilized calcium carbonate)

Hardness of 3-4 on the Mohs scale; colors range from tan, gray, and brown to whitish; luster is dull to vitreous when polished; exhibits distinct honeycomb or star-like corallite patterns on the surface; specific gravity of approximately 2.7.

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed when ancient coral reefs were buried under sediment in shallow tropical seas, typically during the Paleozoic era (Devonian period, around 350-400 million years ago), and gradually replaced by silica or calcite through permineralization.

Uses & applications

Used primarily in lapidary arts, cabochons for jewelry, decorative carvings, and as collectible educational specimens.

Geological facts

Fossil coral is the state stone of Michigan, where a specific type known as Petoskey stone (Hexagonaria percarinata) is commonly found along the shores of Lake Michigan.

Field identification & locations

Easily identified by the characteristic network of hexagonal or circular corallites (tiny pits representing the skeletal cups where the polyps lived). Found globally in sedimentary rock formations, especially ancient limestone beds.

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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

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Fossiliferous Limestone with Crinoid stems

Limestone (CaCO3) containing Crinoidea fossils

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