sedimentary
Petoskey Stone
Hexagonaria percarinata
Hardness: 3-3.5 (Mohs scale); Color: Gray to brown when dry, reveals hexagonal patterns when wet; Luster: Dull to vitreous; Crystal structure: Hexagonal (colonial coral); Cleavage: Indistinct; Specific gravity: 2.6-2.7
- Hardness
- 3-3
- Color
- Gray to brown when dry, reveals hexagonal patterns when wet
- Luster
- Dull to vitreous
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Physical properties
Hardness: 3-3.5 (Mohs scale); Color: Gray to brown when dry, reveals hexagonal patterns when wet; Luster: Dull to vitreous; Crystal structure: Hexagonal (colonial coral); Cleavage: Indistinct; Specific gravity: 2.6-2.7
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) from the fossilized remains of colonial rugose corals in warm, shallow saltwater seas covering Michigan.
Uses & applications
The state rock of Michigan; primarily used for jewelry, lapidary arts, decorative carving, and as high-value geological collectibles.
Geological facts
The pattern consists of individual coral polyps called corallites. The name 'Petoskey' comes from an Ottawa Chief named Pet-O-Sega, meaning 'rising sun' or 'rays of hope'.
Field identification & locations
Identified by its distinctive six-sided 'honeycomb' pattern. Found most abundantly on the shores of Lake Michigan and Lake Huron. It is often camouflaged as a gray pebble until wetted or polished.
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