fossil

Fossil Coral

Rugose Coral or Tabulate Coral (Phylum Cnidaria)

Hardness varies from 3 to 4 on the Mohs scale if calcite, or 6.5 to 7 if silicified (replaced by quartz). Colors range from grey, brown, and tan to white. Luster is typically dull to vitreous when polished. Structure exhibits distinct radial or hexagonal corallite patterns.

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed in ancient shallow marine environments, predominantly during the Paleozoic era (such as the Devonian, Silurian, or Carboniferous periods, 300 to 400+ million years ago), where coral skeletons were preserved and mineralized through permineralization or replacement by silica or calcite.

Uses & applications

Used in lapidary work, making cabochons for jewelry, decorative objects, and as educational or display specimens for fossil collectors.

Geological facts

Fossil coral is the state stone of Michigan (Petoskey stone is a specific type of fossilized rugose coral). Many fossil corals have been completely replaced by agate or chalcedony over millions of years through the process of silicification.

Field identification & locations

Can be identified by the distinct star-like patterns or honeycomb-like tubes (corallites) visible on the weathered or polished surface of the rock. Commonly found in limestone quarries, riverbeds, and sedimentary rock formations worldwide.

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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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