sedimentary
Petrified Rugose Coral (Horn Coral)
Order Rugosa (extinct coral)
Hardness: 3-4 (if calcite) to 7 (if silicified); Color: Tan, beige, brown; Luster: Waxy to dull; Structure: Conical or horn-shaped with radial septa (internal lines); Cleavage: None (conchoidal fracture if agatized)
- Hardness
- 3-4 (if calcite) to 7 (if silicified)
- Color
- Tan, beige, brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 3-4 (if calcite) to 7 (if silicified); Color: Tan, beige, brown; Luster: Waxy to dull; Structure: Conical or horn-shaped with radial septa (internal lines); Cleavage: None (conchoidal fracture if agatized)
Formation & geological history
Formed in warm, shallow marine environments. Most common specimens date from the Ordovician to the Permian periods (approx. 488 to 251 million years ago). The organic material was replaced by minerals like calcite or silica over millions of years through permineralization.
Uses & applications
Primarily used as educational specimens, decorative items, and lapidary material for jewelry if agatized (silicified).
Geological facts
Rugose corals are called 'horn corals' due to their resemblance to a bull's horn. They were solitary organisms that lived on the seafloor but became extinct during the Great Permian Extinction.
Field identification & locations
Identify by looking for a cone or horn shape and radial structural lines (septa) visible on the cross-sections. Commonly found in limestone outcrops, riverbeds, and gravel pits across North America and Europe.
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