sedimentary

Fossilized Rugose Coral (Horn Coral)

Order Rugosa (Calcium Carbonate - CaCO3)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale); Color: Gray, tan, or white; Luster: Dull to earthy; Crystal structure: Trigonal/Hexagonal calcite; Cleavage: None (fossilized structure); Specific gravity: 2.7

Hardness: 3-4 (Mohs scale)
Color: Gray, tan, or white
Luster: Dull to earthy

Identified June 28, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs scale); Color: Gray, tan, or white; Luster: Dull to earthy; Crystal structure: Trigonal/Hexagonal calcite; Cleavage: None (fossilized structure); Specific gravity: 2.7

Formation & geological history

Formed through permineralization of ancient coral skeletons in shallow, warm marine environments during the Paleozoic era (approx. 488 to 251 million years ago).

Uses & applications

Educational purposes, paleontological study, lapidary use (cabochons), and as architectural or landscape decorative stones.

Geological facts

Rugose corals are called 'Horn Corals' because of their characteristic horn-like shape. They were major reef builders before becoming extinct during the Great Permian Extinction.

Field identification & locations

Identify by radial septa (vertical plates) visible in the cross-section and the 'wrinkled' rugose exterior. Commonly found in limestone quarries and riverbeds in the Midwest US and UK.