sedimentary

Fossil Coral (Rugosa or Solitary Coral)

Rugosa (Order), often referred to as 'Horn Coral' (Lithostrotionella or Grewingkia)

AI-generated identification · may be incorrect

Hardness: 3.5-4 (Mohs), though can reach 7 if silicified; Color: Gray, tan, or white; Luster: Dull to earthy; Structure: Cylindrical or conical with radial septa; Gravity: 2.6-2.7.

Hardness: 3
Color: Gray, tan, or white
Luster: Dull to earthy

Identified March 14, 2026More sedimentary →

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Physical properties

Hardness: 3.5-4 (Mohs), though can reach 7 if silicified; Color: Gray, tan, or white; Luster: Dull to earthy; Structure: Cylindrical or conical with radial septa; Gravity: 2.6-2.7.

Formation & geological history

Formed through the fossilization of marine organisms in shallow tropical seas during the Paleozoic era (approx. 488 to 251 million years ago). The organic calcium carbonate structure is replaced by minerals like calcite or silica over geological time.

Uses & applications

Used primarily for educational purposes, museum exhibits, and geological collecting. Petrified or agatized specimens are sometimes polished for jewelry (cabochons) or decorative paperweights.

Geological facts

Rugose corals are extinct. They differ from modern corals by having a unique four-fold symmetry in their septal arrangement. They are nicknamed 'Horn Corals' due to their resemblance to a bull's horn.

Field identification & locations

Identify in the field by looking for the 'septal' pattern (lines radiating from the center) on the cross-section. Commonly found in limestone outcrops in the Midwestern United States (Ohio, Indiana, Kentucky). Collectors should look for well-preserved radial structure.