sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs); Color: Gray when dry, mottled pattern when wet; Luster: Dull to vitreous when polished; Crystal structure: Hexagonal corallites; Cleavage: None; Specific gravity: 2.7

Hardness: 3-4 (Mohs)
Color: Gray when dry, mottled pattern when wet
Luster: Dull to vitreous when polished

Identified June 30, 2026More sedimentary →

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Physical properties

Hardness: 3-4 (Mohs); Color: Gray when dry, mottled pattern when wet; Luster: Dull to vitreous when polished; Crystal structure: Hexagonal corallites; Cleavage: None; Specific gravity: 2.7

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) from the fossilized remains of ancient rugose corals in warm, shallow seas that once covered Michigan.

Uses & applications

State stone of Michigan, used in lapidary work, jewelry making, ornamental carvings, and widely collected by fossil enthusiasts.

Geological facts

The name 'Petoskey' comes from the Ottawa Indian Chief Pet-O-Sega. The stone's distinctive pattern is only visible when the stone is wet or polished.

Field identification & locations

Identified by its distinctive six-sided 'honeycomb' or sunburst pattern. Most commonly found along the shores of Lake Michigan and Lake Huron in the northern lower peninsula of Michigan.