fossil
Coral Fossil (Petoskey-style or Colonial)
Hexagonaria percarinata (Order: Rugosa)
Hardness: 3-4 (Mohs), Color: Grey, beige, or tan, Luster: Dull to earthy when dry, silky when wet, Crystal structure: Hexagonal honeycomb pattern (corallites), Specific gravity: 2.6-2.7
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Physical properties
Hardness: 3-4 (Mohs), Color: Grey, beige, or tan, Luster: Dull to earthy when dry, silky when wet, Crystal structure: Hexagonal honeycomb pattern (corallites), Specific gravity: 2.6-2.7
Formation & geological history
Formed during the Devonian period (approx. 350-400 million years ago) in shallow, warm tropical seas. The skeletal structure of the coral was eventually replaced by minerals like calcite or silica during fossilization.
Uses & applications
Primarily used for lapidary work (polishing), jewelry making, home decor, and as educational geological specimens.
Geological facts
The Michigan state stone, the Petoskey stone, is a specific type of this fossil. The distinctive 'eyes' seen on the stone are the individual cross-sections of the coral polyps.
Field identification & locations
Identify by looking for the honeycomb or 'star' pattern on the surface. They are most easily found on beaches along the Great Lakes (especially Lake Michigan) or in limestone quarries. Wetting the stone often reveals the hidden fossil pattern more clearly.
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