fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3-4 (Mohs scale), Color: Gray/white when dry, distinct honeycomb pattern when wet, Luster: Dull to vitreous, Structure: Hexagonal colonial coral cells, Composition: Calcite

Identified July 6, 2026More fossil →

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Physical properties

Hardness: 3-4 (Mohs scale), Color: Gray/white when dry, distinct honeycomb pattern when wet, Luster: Dull to vitreous, Structure: Hexagonal colonial coral cells, Composition: Calcite

Formation & geological history

Formed approximately 350 million years ago during the Devonian period in shallow tropical seas. They are fossilized colonial rugose corals from the Hexagonaria genus.

Uses & applications

Used in lapidary work for jewelry, home decor, and educational collections. It is the official state stone of Michigan.

Geological facts

These fossils were moved from their original location by glacial action during the last ice age, which is why they are found as rounded cobbles along lake shores.

Field identification & locations

Identified by the unique six-sided 'honeycomb' pattern. They are easiest to find on the beaches of Lake Michigan after a storm or when wet, as the pattern is obscured when the stone is dry.