fossil

Fossilized Horn Coral

Rugosa (Tetracoralla)

Hardness around 3-7 depending on silica or calcite replacement; color varies from tan, brownish-orange, yellow, to gray; displays distinctive ladder-like or segmented internal radial/transverse wall structures (septa and tabulae); dull to resinous or waxy luster.

Identified July 27, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Paleozoic Era (Ordovician to Permian period, roughly 488 to 251 million years ago) in warm, shallow marine environments. The soft coral organic material decomposed, and the skeletal calcium carbonate shell was buried, mineralized, and often replaced by silica (silicified) or calcite.

Uses & applications

Popular among fossil collectors, lapidary artists for cabochons and jewelry, and used as educational specimens to teach historical geology and paleontology.

Geological facts

Rugose corals were solitary organisms that grew a horn-like exoskeleton, earning them the nickname 'horn coral.' They went completely extinct during the Permian-Triassic extinction event about 252 million years ago.

Field identification & locations

Look for repeating, parallel horizontal partition lines (tabulae) or fan-like radial ribs (septa) in a curved tubular shape. Frequently found in limestone beds, gravel pits, and riverbeds across North America (such as the Midwestern US and Michigan).