sedimentary
Fossil Coral (Rugose)
Rugosa (Order), often preserved in Limestone (CaCO3)
Hardness: 3-4 on Mohs scale; Color: light gray to tan; Luster: dull to earthy; Structure: distinct radiating septa and ribbing (costae); Cleavage: none; Specific gravity: approx 2.7.
- Hardness
- 3-4 on Mohs scale
- Color
- light gray to tan
- Luster
- dull to earthy
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Physical properties
Hardness: 3-4 on Mohs scale; Color: light gray to tan; Luster: dull to earthy; Structure: distinct radiating septa and ribbing (costae); Cleavage: none; Specific gravity: approx 2.7.
Formation & geological history
Formed through the fossilization of marine organisms in warm, shallow seas. These rugose corals typically date from the Ordovician to the Permian periods (approx. 488 to 251 million years ago).
Uses & applications
Primarily used for educational purposes, scientific study, and as a collector's item. Larger, high-quality specimens are sometimes used in lapidary work or landscape decoration.
Geological facts
Also known as 'horn corals' due to the characteristic horn-like shape of the solitary species. They are extinct reef-builders that vanished during the great Permian-Triassic extinction event.
Field identification & locations
Identify by looking for the radial symmetry of the septa (inner plates) and the horizontal growth lines on the exterior. Often found in limestone outcrops, riverbeds, and gravel pits across North America (notably the Midwest).
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