sedimentary

Fossil Rugose Coral (Horn Coral)

Order Rugosa (likely Grewingkia or Heliophyllum species); Calcium Carbonate (CaCO3)

Hardness 3 (calcitic replacement); Color: Gray, tan, or earthy brown; Luster: Dull or earthy; Structure: Solitary horn-shaped fossil with radiating internal septa and growth rings (rugae); Specific gravity: ~2.7

Color: Gray, tan, or earthy brown
Luster: Dull or earthy

Identified March 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the fossilization of marine organisms in shallow, warm Paleozoic seas. These specimens typically range from the Ordovician to the Permian periods (~485 to 251 million years ago).

Uses & applications

Used primarily as educational specimens, for paleontological study, and in mineral/fossil collecting. Polished specimens are occasionally used in jewelry or as decorative paperweights.

Geological facts

Rugose corals lived as solitary polyps (nicknamed 'Horn Corals') or in colonies. Unlike modern corals, their skeletons feature prominent ridges (rugae) that represent daily or seasonal growth spurts, which scientists use to calculate the length of the year in deep time.

Field identification & locations

Identify in the field by looking for a cone or 'horn' shape with a hollow or septum-filled circular end. Common in limestone outcrops in the Midwestern USA, Canada, and Europe. Collectors should look in river beds or quarries containing limestone.

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