fossil
Petoskey Stone
Hexagonaria percarinata (Fossilized colonial rugose coral)
Hardness: 3-4 (Mohs scale); Color: Gray to tan, white when dry; Luster: Dull to vitreous; Crystal structure: Amorphous fossil structure; Cleavage: None; Specific gravity: approx. 2.7
- Hardness
- 3-4 (Mohs scale)
- Color
- Gray to tan, white when dry
- Luster
- Dull to vitreous
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Physical properties
Hardness: 3-4 (Mohs scale); Color: Gray to tan, white when dry; Luster: Dull to vitreous; Crystal structure: Amorphous fossil structure; Cleavage: None; Specific gravity: approx. 2.7
Formation & geological history
Formed during the Devonian Period, approximately 350-400 million years ago. These were colonial corals that lived in warm, shallow seas covering what is now Michigan and the Midwest. Over millions of years, the coral was replaced by calcite and buried by sediment.
Uses & applications
Primarily used for jewelry, lapidary art, and as a popular collector's item. In Michigan, it is the official state stone and is often polished to reveal its intricate patterns.
Geological facts
The unique hexagonal pattern is formed by the individual coral polyps. The name 'Petoskey' comes from an Ottawa Indian Chief, Chief Pet-O-Sega, meaning 'rising sun' or 'rays of hope.'
Field identification & locations
Found most commonly on the shores of Lake Michigan and Lake Huron. Identify in the field by looking for gray stones with a distinct 'honeycomb' or 'six-sided' pattern, which becomes much clearer when the stone is wet.
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