sedimentary

Fossil Coral (Petoskey Stone / Rugose Coral)

Hexagonaria percarinata (Order: Rugosa, Fossilized Calcium Carbonate CaCO3)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale); Color: Gray to olive-brown when dry, dark patterns when wet; Luster: Dull/Earthy; Crystal Structure: Hexagonal skeletal chambers; Cleavage: None/Fragmentary.

Hardness: 3-4 (Mohs scale)
Color: Gray to olive-brown when dry, dark patterns when wet
Luster: Dull/Earthy

Identified March 21, 2026More sedimentary →

Explore Fossil Coral (Petoskey Stone / Rugose Coral) 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 (Mohs scale); Color: Gray to olive-brown when dry, dark patterns when wet; Luster: Dull/Earthy; Crystal Structure: Hexagonal skeletal chambers; Cleavage: None/Fragmentary.

Formation & geological history

Formed through the fossilization of ancient coral reefs. These specimens date back to the Devonian period (approx. 350-400 million years ago) when shallow seas covered the Earth's surface.

Uses & applications

Used primarily for jewelry making, lapidary arts (polishing), souvenirs, and as educational geological specimens.

Geological facts

The 'Petoskey stone' is the state stone of Michigan. The unique 'six-sided star' pattern only reveals itself clearly when the stone is wet or polished to a high gloss.

Field identification & locations

Identify in the field by looking for a honeycomb or 'brain' pattern on the surface. Commonly found on the beaches of the Great Lakes (especially Lake Michigan) and in limestone quarries. Collectors should look for wet stones to spot the pattern easily.