fossil

Petoskey Stone (Fossilized Coral)

Hexagonaria percarinata

Hardness: 3-4 on Mohs scale; Color: Grayish-tan, brownish to dark gray; Luster: Dull to vitreous when wet or polished; Structure: Calcite replacement showing distinct 6-sided hexagonal coral skeleton patterns (corallites).

Hardness: 3-4 on Mohs scale
Color: Grayish-tan, brownish to dark gray
Luster: Dull to vitreous when wet or polished

Identified July 26, 2026More fossil →

Explore Petoskey Stone (Fossilized Coral) 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 during the Devonian Period (around 350 million years ago) when Michigan was covered by a warm, shallow sea. Ancient rugose corals died, were buried by sediment, and were mineralized primarily with calcite.

Uses & applications

Michigan's official state stone, popular for lapidary work, jewelry, stone carving, decorative display, and amateur fossil collecting.

Geological facts

When dry, a Petoskey stone looks like ordinary limestone, but when wet or polished, the intricate hexagonal pattern of the ancient coral polyps magically appears.

Field identification & locations

Identified by the dark central eye (the mouth of the polyp) surrounded by radiating lines within six-sided chambers. Commonly found along beaches of Lake Michigan and Lake Huron.

Other fossil specimens

See all

Crinoid Stem Fossil in Matrix

Phylum Echinodermata, Class Crinoidea

fossil

Sedimentary Outcrop (Sandstone/Shale)

Sedimentary bedrock formation (variable composition)

sedimentary

Chert with Fossil Brachiopod

Microcrystalline Quartz (SiO2) with Calcitic Brachiopod inclusion

sedimentary

Fossiliferous Limestone

Fossiliferous Calcilutite / Biomicrite

sedimentary

Fossilized Crinoid Stem in Limestone

Crinoidea (Class) in Calcium Carbonate (CaCO3) matrix

fossil

Fossiliferous Limestone

Limestone with embedded fossil cross-section

sedimentary