sedimentary

Fossil Coral (Petoskey-style Rugose Coral)

Hexagonaria percarinata (Order: Rugosa)

AI-generated identification · may be incorrect

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

Identified June 16, 2026More sedimentary →

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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.