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

Petoskey Stone (Fossilized Coral)

Hexagonaria percarinata

AI-generated identification · may be incorrect

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 More fossil →
Explore Petoskey Stone (Fossilized 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; 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).

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.

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
Fossil oyster shell (Oyster (family Ostreidae); shell composed chiefly of calcite (CaCO₃)) — fossil

Fossil oyster shell

Oyster (family Ostreidae); shell composed chiefly of calcite (CaCO₃)

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

Rounded sedimentary pebble

Likely micritic limestone or calcareous sandstone

sedimentary