fossil

Fossil Rugose Coral (Horn Coral)

Order Rugosa

Hardness: 3-4 (calcite) or 7 (if silicified); Color: tan, grey, or black; Luster: dull to earthy; Structure: radial septa in a circular cross-section; Specific Gravity: 2.7

Hardness: 3-4 (calcite) or 7 (if silicified)
Color: tan, grey, or black
Luster: dull to earthy

Identified July 2, 2026More fossil →

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Physical properties

Hardness: 3-4 (calcite) or 7 (if silicified); Color: tan, grey, or black; Luster: dull to earthy; Structure: radial septa in a circular cross-section; Specific Gravity: 2.7

Formation & geological history

Formed in warm, shallow marine environments during the Ordovician to Permian periods (approx. 488 to 251 million years ago). The specimen represents a horizontal cross-section of a solitary coral's skeleton.

Uses & applications

Primarily for educational purposes, museum displays, and private fossil collections. Also used as decorative garden stones or in jewelry when silicified (agatyzed).

Geological facts

Rugose corals are extinct. Unlike modern corals, they grew a hard outer skeleton of calcite. They are often called 'Horn Corals' because their solitary forms often grew in a curved, horn-like shape.

Field identification & locations

Identify by the distinct radial symmetry (lines radiating from the center, called septa). Commonly found in limestone outcrops, riverbeds, and Great Lakes beaches (like the Petoskey stone, which is a colonial rugose coral).