sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3.0-3.5 (Mohs scale), Color: Gray to tan (reveals unique hexagonal patterns when wet), Luster: Dull to vitreous when polished, Crystal structure: Hexagonal corallites with radiating septa, Specific gravity: 2.7

Hardness: 3

Identified June 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the fossilization of colonial rugose coral (Hexagonaria) that lived in warm, shallow Devonian seas approximately 350 million years ago. Over time, the calcareous skeleton was replaced by calcite or silica and smoothed by glacial movement and water erosion.

Uses & applications

Primarily used as a state stone for Michigan, for jewelry making (pendants, rings), decorative ornaments, stone carving, and as a popular collector's item.

Geological facts

The Petoskey stone is the state stone of Michigan. The name comes from an Ottawa Chief named Pet-O-Sega, and the unique hexagonal pattern is often called a 'sunray' pattern. When dry, it looks like ordinary limestone, but the pattern pops when wet or polished.

Field identification & locations

Identify in the field by looking for a characteristic honeycomb or hexagonal 'eye' pattern on stones along beaches, especially after rain or when submerged. Commonly found on the shores of Lake Michigan and Lake Huron.

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