fossil

Petoskey Stone

Hexagonaria percarinata (Fossilized rugose coral)

Mohs hardness of 3 to 4 (due to calcite composition), color ranges from light gray to brown, vitreous to dull luster, hexagonal corallites with distinct dark centers, specific gravity of approximately 2.7.

Identified August 4, 2026More fossil →

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 during the Devonian period, approximately 350 million years ago, when Michigan was covered by a warm, shallow tropical sea. The coral skeletons were fossilized and later transported by glacial activity.

Uses & applications

Used primarily in lapidary arts for crafting jewelry, decorative objects, and as a popular collectible specimen. It is the official state stone of Michigan.

Geological facts

Petoskey stones are a unique type of fossilized coral where the coral's corallites appear as distinct six-sided eyes when polished. They become much more vivid and easy to spot when wet.

Field identification & locations

Easily identified in the field by the characteristic pattern of hexagonal shapes with radiating lines and dark centers. Most commonly found along the shores of Lake Michigan and Lake Huron, as well as in gravel pits across northern Michigan.

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

Crinoid Stem Fossils (Indian Beads / St. Cuthbert's Beads)

Crinoid columnal ossicles (phylum Echinodermata)

fossil

Fossilized Crinoid Stem in Limestone

Crinoidea (Class) in Calcium Carbonate (CaCO3) matrix

fossil

Fossiliferous Limestone

Limestone with embedded fossil cross-section

sedimentary