sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3-4 (Mohs scale); Color: Gray to tan when dry, distinct hexagonal patterns visible when wet; Luster: Dull to earthy (rough), vitreous (polished); Structure: Hexagonal corallites with radiating septa; Composition: Calcite/Limestone.

Hardness: 3-4 (Mohs scale)
Luster: Dull to earthy (rough), vitreous (polished)

Identified May 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago). These are fossilized colonial corals that lived in warm, shallow seas covering what is now Michigan. Over time, the coral was buried in sediment and replaced by calcite.

Uses & applications

Popular in jewelry making, lapidary arts, decorative souvenirs, and as a highly sought-after collector items for fossil enthusiasts.

Geological facts

It is the state stone of Michigan. The name comes from an Ottawa Chief named Petosegay. The patterns are easiest to see when the stone is wet or polished, revealing the unique 'sunburst' or hexagonal 'honeycomb' structure of the ancient coral.

Field identification & locations

Identify in the field by looking for gray, rounded pebbles on beaches (especially Lake Michigan) that show a distinctive hexagonal pattern when splashed with water. They are primarily found in the Traverse Group of Devonian age rocks.

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