sedimentary
Fossilized Rugose Coral (Horn Coral)
Order Rugosa (Calcium Carbonate - CaCO3)
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
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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.
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