fossil

Fossil Coral

Hexagonaria or Rugose Coral (Fossilized Calcium Carbonate / Silica)

Hardness of 3-4 on Mohs scale (if calcified) or 6.5-7 (if silicified); Colors range from beige, brown, tan to grey and white; Luster is dull to vitreous; Often displays distinct internal corallite structures like honeycomb patterns or septa.

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed in warm, shallow marine environments millions of years ago (often Paleozoic era, around 350-400 million years ago) when ancient coral reefs were buried under sediment and progressively replaced by minerals like silica or calcite through permineralization.

Uses & applications

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

Geological facts

Fossil coral is the state stone of Michigan (specifically the Petoskey stone, which is a fossilized rugose coral). The distinct patterns visible on the surface are the skeletal remains of ancient marine colonial organisms.

Field identification & locations

Can be identified in the field by looking for characteristic geometric honeycomb, star-like, or parallel tube patterns on weathered rock surfaces. Commonly found in sedimentary limestone formations worldwide.

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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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