sedimentary
Hexagonaria (Petoskey Stone)
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-3.5 (Mohs), Color: Gray to beige (dry) or mottled dark brown (wet/polished), Luster: Dull to earthy when raw, vitreous when polished, Crystal structure: Hexagonal skeletal chambers, Specific gravity: 2.7
- Hardness
- 3-3
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Physical properties
Hardness: 3-3.5 (Mohs), Color: Gray to beige (dry) or mottled dark brown (wet/polished), Luster: Dull to earthy when raw, vitreous when polished, Crystal structure: Hexagonal skeletal chambers, Specific gravity: 2.7
Formation & geological history
Formed during the Devonian period, roughly 350 million years ago. These were colonial corals living in warm, shallow tropical seas that once covered what is now the midwestern United States. Over geological time, the coral skeleton was replaced by calcite and silica.
Uses & applications
Commonly used for decorative ornamental jewelry, carving, and as a popular state stone for collectors. In the past, it was also used as a source of architectural limestone.
Geological facts
It is the official state stone of Michigan. The name 'Petoskey' comes from an Ottawa Indian chief, Ignatius Petosega, whose name means 'rays of the rising sun'. The distinctive pattern is the fossilized anatomy of the coral polyps.
Field identification & locations
Identify in the field by the distinct hexagonal 'honeycomb' pattern that becomes highly visible when the stone is wet. Found primarily along the shores of Lake Michigan and Lake Huron. Collectors should look for water-worn pebbles on beaches.
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