fossil

Fossilized Horn Coral

Rugosa (Rugose coral)

Hardness: 3-3.5 on Mohs scale (calcite replacement) or ~7 (silicified/quartz replacement); Color: brownish-tan, grey, to black; Luster: earthy to dull; Structure: fan-like or radial internal septa radiating from the center.

Hardness: 3-3
Color: brownish-tan, grey, to black
Luster: earthy to dull

Identified July 27, 2026More fossil →

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Physical properties

Formation & geological history

Formed in ancient warm, shallow seas during the Ordovician through Permian periods (roughly 488 to 251 million years ago) as marine invertebrates that created calcium carbonate skeletons, which later permineralized.

Uses & applications

Popular among fossil collectors, educational specimens, and occasionally polished for use in decorative crafts or cabochons.

Geological facts

Horn corals gets their common name from their horn-like solitary shape. The rugose corals went extinct during the massive Permian-Triassic extinction event around 252 million years ago.

Field identification & locations

Identified in the field by its distinct cross-sectional radial septa pattern resembling rays or fan blades. Commonly found in limestone beds, gravel bars, and creek beds throughout the Midwestern United States and ancient marine basins globally.