fossil

Fossil Coral (Petoskey Stone or Chain Coral Pebble)

Hexagonaria percarinata / Rugosa coral (CaCO3)

AI-generated identification · may be incorrect

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

Identified July 24, 2026More fossil →

Explore Fossil Coral (Petoskey Stone or Chain Coral Pebble) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

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.