fossil

Fossilized Horn Coral

Rugosa (Order)

Hardness: 3-3.5 on Mohs scale (calcite matrix); Color: Grayish-brown, tan, to dull buff; Luster: Dull to earthy; Structure: Radiating septa visible along the outer edges and cross-section; Specific Gravity: ~2.7

Hardness: 3-3
Color: Grayish-brown, tan, to dull buff
Luster: Dull to earthy

Identified July 26, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Paleozoic era (Ordovician to Permian, ~488 to 251 million years ago) in shallow, warm marine environments where solitary rugose corals grew on the sea floor before being buried in marine sediments and mineralized over millions of years.

Uses & applications

Popular among fossil collectors, used in educational science kits, lapidary arts, and sometimes polished into decorative cabochons or worry stones.

Geological facts

Rugose corals are extinct horn corals that lived as solitary polyps. Unlike modern corals, they produced a calcitic skeleton with distinct septal patterns. They completely perished during the end-Permian mass extinction event.

Field identification & locations

Identified by its distinctive horn-like or disc-like shape featuring radial lines (septa) extending outward from a central point. Commonly found in Paleozoic limestone and shale deposits worldwide, including the Midwestern United States (such as Michigan, Indiana, and Ohio).