sedimentary

Petoskey Stone

Hexagonaria percarinata

Hardness: 3.0-3.5 on Mohs scale; Color: Gray to beige with distinctive hexagonal pattern; Luster: Matte to waxy; Crystal structure: Calcite; Cleavage: Perfect rhombohedral; Specific gravity: 2.7.

Hardness: 3
Color: Gray to beige with distinctive hexagonal pattern
Luster: Matte to waxy

Identified May 31, 2026More sedimentary →

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Physical properties

Hardness: 3.0-3.5 on Mohs scale; Color: Gray to beige with distinctive hexagonal pattern; Luster: Matte to waxy; Crystal structure: Calcite; Cleavage: Perfect rhombohedral; Specific gravity: 2.7.

Formation & geological history

Formed from fossilized rugose coral during the Devonian period (roughly 350 million years ago) in shallow, warm seawater. Subsequent glaciation distributed the coral into limestone deposits across the northern Lower Peninsula of Michigan.

Uses & applications

Primarily used for decorative purposes, jewelry (pendants and beads), lapidary art, and as a popular souvenir/collectible specimen.

Geological facts

It is the state stone of Michigan and is technically a colony of fossilized coral rather than a simple rock. When dry, it often looks like ordinary limestone, but the pattern becomes vivid when wet.

Field identification & locations

Identify in the field by wetting the stone to reveal the 'sunburst' or hexagonal coral pattern. Most commonly found along the shores of Lake Michigan and Lake Huron. Large or perfectly patterned stones are highly desirable for collectors.