fossil
Fossil Coral (Petoskey Stone or Chain Coral Pebble)
Hexagonaria percarinata / Rugosa coral (CaCO3)
Hardness: 3-4 on Mohs scale; Color: Light gray to tan with distinct white net-like pattern; Luster: Dull to earthy when unpolished; Cleavage: None (calcite fossil replacement)
- Hardness
- 3-4 on Mohs scale
- Color
- Light gray to tan with distinct white net-like pattern
- Luster
- Dull to earthy when unpolished
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Physical properties
Hardness: 3-4 on Mohs scale; Color: Light gray to tan with distinct white net-like pattern; Luster: Dull to earthy when unpolished; Cleavage: None (calcite fossil replacement)
Formation & geological history
Formed during the Devonian period (around 350-400 million years ago) from ancient marine coral reefs whose structures were mineralized by calcite and silica before being smoothed by glacial action and wave erosion.
Uses & applications
Popular as a collector's fossil pebble, lapidary material for polishing into cabochons, decorative stones, and souvenir craft items.
Geological facts
The characteristic net or hexagonal spiderweb-like pattern consists of individual coral polyps (corallites) that formed colony structures in prehistoric shallow tropical seas.
Field identification & locations
Identify by looking for pale honeycomb or mesh patterns against a grey matrix on water-worn beach pebbles. Commonly found along the shores of Lake Michigan and Great Lakes gravel deposits.
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