sedimentary
Fossil Coral (Rugose Coral)
Rugosa (extinct order); typically composed of Calcite (CaCO3) or replaced by Quartz (SiO2)
Hardness: 3 (if calcite) to 7 (if silicified); Color: Gray, tan, or white; Luster: Dull to earthy; Structure: Visible longitudinal septa (lines) and growth rings; Cleavage: None (conchoidal fracture if agatized).
- Hardness
- 3 (if calcite) to 7 (if silicified)
- Color
- Gray, tan, or white
- Luster
- Dull to earthy
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Physical properties
Hardness: 3 (if calcite) to 7 (if silicified); Color: Gray, tan, or white; Luster: Dull to earthy; Structure: Visible longitudinal septa (lines) and growth rings; Cleavage: None (conchoidal fracture if agatized).
Formation & geological history
Formed as the exoskeletons of solitary or colonial marine polyps in warm, shallow seas. These rugose corals typically date from the Ordovician period to the Permian period (approx. 488 to 251 million years ago).
Uses & applications
Primarily used for geological study, educational collections, and ornamental lapidary work (if agatized). Sometimes used as decorative gravel or in cement making if found in large limestone deposits.
Geological facts
Rugose corals are often called 'horn corals' due to their shape. They went extinct during the 'Great Dying'—the end-Permian mass extinction event—and are distinct from modern corals by their bilateral symmetry.
Field identification & locations
Identify in the field by looking for the characteristic 'ribbed' or lined exterior texture in limestone-rich areas. Common in the Great Lakes region (e.g., Petoskey stones are a type of rugose coral) and the Ohio River Valley.
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