sedimentary
Fossilized Rugose Coral (Horn Coral)
Order Rugosa (extinct tetra-coral)
Hardness: 3-4 (calcite composition), Color: タン (tan), gray, or light brown, Luster: Dull to earthy, Crystal Structure: Radiating septa visible in cross-section, specific gravity: 2.7
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Physical properties
Hardness: 3-4 (calcite composition), Color: タン (tan), gray, or light brown, Luster: Dull to earthy, Crystal Structure: Radiating septa visible in cross-section, specific gravity: 2.7
Formation & geological history
Formed in shallow marine environments during the Paleozoic era (Ordovician to Permian periods). These corals were solitary or colonial and were preserved through permineralization or replacement by calcite.
Uses & applications
Primarily used as educational specimens, collectibles, and occasionally in jewelry or as decorative stones in masonry.
Geological facts
Rugose corals are known as 'horn corals' due to the horn-like shape of many solitary species. They went extinct during the Great Permian Extinction approximately 250 million years ago.
Field identification & locations
Identified by the distinct vertical ridges (septa) and the cone or cup-like shape (calyx). Commonly found in limestone outcrops or stream beds across the Midwest United States and Europe.
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