sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3.5 (Mohs); Color: Grey to tan (brownish when wet); Luster: Matte when raw, vitreous when polished; Crystal Structure: Hexagonal fossil shapes; Cleavage: None (fossilized structure); Specific Gravity: 2.7

Hardness: 3
Color: Grey to tan (brownish when wet)
Luster: Matte when raw, vitreous when polished

Identified June 8, 2026More sedimentary →

Explore Petoskey Stone in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed from the fossilization of colonial coral during the Devonian Period (approximately 350 million years ago) in shallow, warm tropical seas.

Uses & applications

Primarily used for jewelry, decorative carvings, souvenirs, and architectural accents. It is highly prized by rockhounds and collectors.

Geological facts

It is the state stone of Michigan. The unique 'eye' pattern is only visible when the stone is wet or polished because of the way the coral structures trap water or accept a finish.

Field identification & locations

Identify in the field by looking for a distinctive hexagonal pattern on grey pebbles, especially near the substrate of Lake Michigan beaches. Best found after a storm or in the spring when ice has moved stones to the shore.