Rock Identifier
Petoskey Stone (Fossilized Rugose Coral) (Hexagonaria percarinata) — fossil
fossil

Petoskey Stone (Fossilized Rugose Coral)

Hexagonaria percarinata

AI-generated identification · may be incorrect

Hardness 3-4 on Mohs scale, grey to brownish-grey color, dull to waxy luster, composed primarily of calcite (CaCO3). Displays a characteristic hexagonal pattern of fossilized coral polyp chambers when wet or polished.

Identified More fossil →
Explore Petoskey Stone (Fossilized Rugose Coral) in the encyclopedia →

Identify your own rocks.

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

App StoreGoogle Play

Physical properties

Hardness 3-4 on Mohs scale, grey to brownish-grey color, dull to waxy luster, composed primarily of calcite (CaCO3). Displays a characteristic hexagonal pattern of fossilized coral polyp chambers when wet or polished.

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) when Michigan was covered by a warm, shallow sea. Ancient rugose coral colonies were buried, petrified, and transformed into calcite over millions of years, then eroded by glaciers.

Uses & applications

Michigan's official state stone, highly popular for lapidary work, jewelry making, polished specimens, paperweights, and souvenirs.

Geological facts

Named after Chief Pet-O-Sega, an Ottawa Indian chief whose name means 'rays of rising sun', referencing the sunburst pattern seen in the hexagonal coral cells.

Field identification & locations

When dry, it appears as a plain grey rock; wetting or polishing reveals the distinctive hexagonal coral structures. Commonly found along the shores of Lake Michigan and Lake Huron, especially around Petoskey, Michigan.

More like this

Other fossil specimens

Probable fossil-bearing rock or mineral concretion (Unidentified carbonate-rich concretion with a possible fossil inclusion) — fossil

Probable fossil-bearing rock or mineral concretion

Unidentified carbonate-rich concretion with a possible fossil inclusion

fossil
Likely fossil oyster shell (Fossilized bivalve shell; exact species cannot be identified from the image) — fossil

Likely fossil oyster shell

Fossilized bivalve shell; exact species cannot be identified from the image

fossil
Crinoid Stem Fossil in Matrix (Phylum Echinodermata, Class Crinoidea) — fossil

Crinoid Stem Fossil in Matrix

Phylum Echinodermata, Class Crinoidea

fossil
Fossiliferous limestone (likely a crinoidal or shell-fragment limestone) (Bioclastic limestone, composed chiefly of calcite (CaCO₃)) — sedimentary

Fossiliferous limestone (likely a crinoidal or shell-fragment limestone)

Bioclastic limestone, composed chiefly of calcite (CaCO₃)

sedimentary
Rounded sedimentary pebble (Likely micritic limestone or calcareous sandstone) — sedimentary

Rounded sedimentary pebble

Likely micritic limestone or calcareous sandstone

sedimentary
Chert with Fossil Brachiopod (Microcrystalline Quartz (SiO2) with Calcitic Brachiopod inclusion) — sedimentary

Chert with Fossil Brachiopod

Microcrystalline Quartz (SiO2) with Calcitic Brachiopod inclusion

sedimentary