sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3.0-3.5 (Mohs scale); Color: Gray to tan when dry, reveals distinctive six-sided 'sunburst' or 'star' patterns when wet; Luster: Dull to vitreous; Crystal Structure: Microcrystalline calcite; Specific Gravity: 2.7

Hardness: 3
Luster: Dull to vitreous

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approximately 350 million years ago). These are fossilized rugose corals that lived in warm, shallow saltwater seas. Over time, the calcareous structures were buried and replaced by silica and calcite through mineralization.

Uses & applications

Predominantly used for jewelry making (pendants, beads), ornamental carving, state souvenirs, and high-interest geological collecting.

Geological facts

It is the official state stone of Michigan. The name 'Petoskey' comes from an Ottawa Indian Chief, Pet-O-Sega, meaning 'rising sun', 'rays of dawn', or 'sunbeams of promise', referring to the stone's unique pattern.

Field identification & locations

Identify in the field by looking for a 'honeycomb' or 'six-sided star' pattern on gray stones along shorelines. They are most easily spotted when wet. Found commonly along the shores of Lake Michigan and Lake Huron. Collectors should look for distinct, unbroken hexagonal coral cell walls.

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