sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-3.5 on Mohs scale; Color: typically gray to tan when dry, reveals hexagonal patterns when wet; Luster: vitreous to dull; Crystal structure: hexagonal skeletal structure; Specific gravity: approx 2.7
- Hardness
- 3-3
- Luster
- vitreous to dull
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Physical properties
Hardness: 3-3.5 on Mohs scale; Color: typically gray to tan when dry, reveals hexagonal patterns when wet; Luster: vitreous to dull; Crystal structure: hexagonal skeletal structure; Specific gravity: approx 2.7
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago). These are fossilized rugose corals that lived in warm, shallow seas covering what is now the Michigan Basin. Glaciation later plucked them from the bedrock and spread them across the region.
Uses & applications
Primarily used for jewelry, lapidary arts (cabochons), and as a popular souvenir or collector's item in the Great Lakes region.
Geological facts
It is the official state stone of Michigan. The name comes from an Ottawa Chief, Petosegay. The unique 'eye' pattern is actually the skeleton of individual coral polyps.
Field identification & locations
Identify in the field by looking for the distinctive six-sided 'honeycomb' pattern, which becomes much more visible when the stone is wet. They are most commonly found on the beaches of Lake Michigan and Lake Huron, particularly near the city of Petoskey.
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