sedimentary
Fossilized Rugose Coral (Horn Coral)
Rugosa (Order) - often Grewingkia or Zaphrenthis
Hardness: 3-4 (Calcite composition); Color: Tan, brown, or gray; Luster: Dull to earthy; Shape: Characteristic conical or horn-shaped structure with septal ridges inside the cup (calice).
- Hardness
- 3-4 (Calcite composition)
- Color
- Tan, brown, or gray
- Luster
- Dull to earthy
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Physical properties
Hardness: 3-4 (Calcite composition); Color: Tan, brown, or gray; Luster: Dull to earthy; Shape: Characteristic conical or horn-shaped structure with septal ridges inside the cup (calice).
Formation & geological history
Marine sedimentary environments from the Ordovician to the Permian periods (approx. 488 to 251 million years ago). Formed via the calcification of solitary ahermatypic coral polyps in ancient seas.
Uses & applications
Primarily for paleontological study, educational collecting, and occasionally used in jewelry or as decorative pocket stones.
Geological facts
Rugose corals are known as 'horn corals' because of their unique horn-like shape. They went extinct during the Great Permian Extinction, making them an index fossil for Paleozoic strata.
Field identification & locations
Identify by the cone shape and the radial symmetry (septa) visible in the cross-section. Commonly found in limestone outcrops, creek beds, and quarries across the Midwestern United States and UK.
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