fossil

Fossil Coral (Petoskey Stone / Rugose Coral)

Hexagonaria percarinata (or similar fossilized rugose/tabulate coral)

Hardness: 4 to 5 on the Mohs scale (due to silicification or calcification). Color: Tan, brown, or grey with distinctive hexagonal or circular corallite patterns. Luster: Dull to vitreous when polished. Crystal structure: Microcrystalline quartz or calcite replacement.

Luster: Dull to vitreous when polished

Identified July 31, 2026More fossil →

Explore Fossil Coral (Petoskey Stone / Rugose Coral) in the encyclopedia →

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Physical properties

Formation & geological history

Formed during the Devonian period (approximately 350 to 400 million years ago) when shallow warm seas covered the regions, and coral colonies became fossilized and often replaced by minerals like silica or calcite over millions of years.

Uses & applications

Used primarily in lapidary arts for creating polished decorative stones, jewelry, paperweights, and collectable geological specimens.

Geological facts

Fossil corals like the Petoskey stone are state stones in certain regions. The distinct patterns are the fossilized remains of the skeleton of ancient colonial marine organisms called rugose or tabulate corals.

Field identification & locations

Easily identified by the honeycomb or patterned tubular impressions on the surface. Commonly found in glaciated gravel deposits, ancient limestone formations, and riverbeds in the Great Lakes region of North America and various global limestone deposits.

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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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