fossil

Horn Coral (Rugose Coral)

Order Rugosa (extinct)

Hardness: 3-4 (Calcite); Color: Gray, tan, or brown; Luster: Dull or earthy; Shape: Characteristic horn-like or cone-like shape with septa (radiating lines) often visible in cross-section.

Hardness: 3-4 (Calcite)
Color: Gray, tan, or brown
Luster: Dull or earthy

Identified July 20, 2026More fossil →

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Physical properties

Hardness: 3-4 (Calcite); Color: Gray, tan, or brown; Luster: Dull or earthy; Shape: Characteristic horn-like or cone-like shape with septa (radiating lines) often visible in cross-section.

Formation & geological history

Formed from the calcium carbonate skeletons of solitary coral polyps that lived on the seafloor. Geological age ranges from the Ordovician to the Permian (roughly 488 to 251 million years ago).

Uses & applications

Primarily of value to fossil collectors, educators, and researchers. Occasionally used in jewelry if the fossil has been replaced by agate or jasper.

Geological facts

Horn corals are solitary animals, unlike modern reef-building corals. They are extinct, having died out during the Great Permian Extinction, the largest mass extinction in Earth's history.

Field identification & locations

Identify by looking for a cone or 'horn' shape with internal radial segments. Commonly found in limestone outcrops, road cuts, and streambeds, particularly in the Midwestern United States.