sedimentary
Fossil Coral (Rugose Coral)
Rugosa (extinct order of solitary and colonial corals)
Hardness: 3-4 (Calcite-based); Color: Tan, beige, or gray; Luster: Dull to earthy; Structure: Visible septa (radiating lines) and growth rings; Cleavage: None (fragmented); Specific Gravity: 2.7.
- Hardness
- 3-4 (Calcite-based)
- Color
- Tan, beige, or gray
- Luster
- Dull to earthy
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Physical properties
Hardness: 3-4 (Calcite-based); Color: Tan, beige, or gray; Luster: Dull to earthy; Structure: Visible septa (radiating lines) and growth rings; Cleavage: None (fragmented); Specific Gravity: 2.7.
Formation & geological history
Formed in warm, shallow marine environments during the Paleozoic era (approx. 488 to 251 million years ago). They were buried in calcium-rich sediment which lithified into limestone over time.
Uses & applications
Educational specimens, minor lapidary use if silica-replaced (Petoskey stones), geological dating, and decorative collections.
Geological facts
Commonly known as 'horn corals' because of their unique cone-like shape. They went extinct at the end of the Permian period during the 'Great Dying' mass extinction event.
Field identification & locations
Identify by radiating lines (septa) within a circular or horn-shaped cross-section. Often found in limestone outcrops, riverbeds, and gravel pits across the Midwestern United States and Europe.
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