sedimentary
Fossil Coral (Petoskey-style Rugose Coral)
Hexagonaria percarinata (Order: Rugosa)
Hardness: 3-4 (calcite) to 6.5 (if silicified); Color: Gray, tan, or brown; Luster: Dull/earthy; Structure: Colonial coral with hexagonal or radiating septa; Cleavage: None (fragmented).
- Hardness
- 3-4 (calcite) to 6
- Color
- Gray, tan, or brown
- Luster
- Dull/earthy
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Physical properties
Hardness: 3-4 (calcite) to 6.5 (if silicified); Color: Gray, tan, or brown; Luster: Dull/earthy; Structure: Colonial coral with hexagonal or radiating septa; Cleavage: None (fragmented).
Formation & geological history
Formed through the fossilization of colonial coral reefs during the Devonian Period (approx. 350-400 million years ago) in shallow, warm tropical seas.
Uses & applications
Used in decorative items, jewelry (specifically polished Petoskey stones), ornamental carvings, and as a popular educational/collecting specimen.
Geological facts
The most famous version is the Petoskey stone, the state stone of Michigan. The radiating patterns are the internal skeletal structures of the coral called septa.
Field identification & locations
Identify by the repeating honeycombed or 'sunburst' pattern on the surface. Commonly found on Great Lakes beaches, particularly Lake Michigan, or in gravel pits throughout the Midwest.
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