sedimentary

Fossilized Horn Coral

Order Rugosa (Calcium Carbonate - CaCO3)

Hardness: 3-4 (calcite) to 7 (if silicified); Color: Tan, gray, or white; Luster: Dull to vitreous; Structure: Solitary cup-like skeleton with radial septa (lines) visible in cross-section; Specific Gravity: 2.7

Hardness: 3-4 (calcite) to 7 (if silicified)
Color: Tan, gray, or white
Luster: Dull to vitreous

Identified June 27, 2026More sedimentary →

Explore Fossilized Horn Coral in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed in warm, shallow marine environments during the Paleozoic Era (approx. 488 to 251 million years ago). The specimen is a fossilized skeleton of a solitary coral polyp that was buried in sediment and mineralized over time.

Uses & applications

Primarily used for educational study, geological research, and as collector specimens. Highly silicified pieces are sometimes tumbled or cut for jewelry (cabochons).

Geological facts

Rugose corals are called 'horn corals' because of their distinctive horn-like shape. Unlike modern corals, they were mostly solitary rather than colonial. They went extinct during the Great Permian Extinction event.

Field identification & locations

Identified by the radial septa (rib-like structures) and the cone or cup shape. Commonly found in limestone outcrops, road cuts, and creek beds throughout the Midwestern United States (e.g., Ohio, Indiana, Kentucky).

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary