fossil

Fossil Coral

Hexagonaria (or Rugose/Tabulate coral fossilized in calcium carbonate)

Hardness of 3 on the Mohs scale (due to calcite/aragonite composition), typically gray, tan, or whitish in color, dull to vitreous luster, hexagonal or tubular corallite structures, specific gravity around 2.7.

Identified August 5, 2026More fossil →

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Physical properties

Formation & geological history

Formed in ancient shallow marine environments, typically during the Paleozoic era (such as the Devonian period, around 350-400 million years ago), where coral skeletons accumulated and were later replaced or preserved by silica or calcite during fossilization.

Uses & applications

Used primarily as a collector's specimen, polished into cabochons for jewelry, or displayed as decorative fossil stones.

Geological facts

Fossil coral is the state stone of Michigan (specifically the Petoskey stone, a fossilized colonial rugose coral). The distinct cellular patterns reveal the living chambers (corallites) of ancient marine animals.

Field identification & locations

Easily identified by its distinctive honeycomb or cellular surface pattern. Found worldwide in ancient limestone deposits, particularly in regions with extensive Devonian-aged bedrock. Look for them in gravel beds, quarries, and eroded shorelines.

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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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Crinoid Stem Fossils (Indian Beads / St. Cuthbert's Beads)

Crinoid columnal ossicles (phylum Echinodermata)

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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