fossil

Fossilized Horn Coral

Rugosa (Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3-4 (calcite composition) or 7 (if replaced by silica/agate). Color: Grey, tan, whitish-brown, to dark brownish black. Luster: Dull to pearly, silken or glassy. Structure: Curved horn shape displaying fine radiating septa internal patterns.

Hardness: 3-4 (calcite composition) or 7 (if replaced by silica/agate)
Color: Grey, tan, whitish-brown, to dark brownish black
Luster: Dull to pearly, silken or glassy

Identified July 22, 2026More fossil →

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Physical properties

Formation & geological history

Horn corals are extinct solitary corals that lived on the ancient sea floor from the Ordovician to the Permian Period (488 to 251 million years ago). Over millions of years, the organic structures dissolved and were mineralized by calcite or silica-rich fluids in sedimentary basins.

Uses & applications

Popular among fossil collectors, used in lapidary work (when silicified), educational tools, and decorative displays.

Geological facts

Horn corals get their common name from their unique horn-like conical shape. They were extinct during the Permian-Triassic extinction event, the greatest mass extinction in Earth's history.

Field identification & locations

Identified by its distinctive curved cone or horn shape with longitudinal ridges and radiating internal septa lines. Very common in Paleozoic limestone deposits, gravel bars, and creek beds in regions like the American Midwest.