sedimentary
Petoskey Stone (Fossilized Coral)
Hexagonaria percarinata (CaCO3)
Hardness: 3.0-3.5 (Mohs); Color: Gray to light brown, revealing a distinct hexagonal pattern when wet; Luster: Dull to pearly; Crystal structure: Orthorhombic (as Calcite substitution); Specific Gravity: 2.7.
- Hardness
- 3
- Luster
- Dull to pearly
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Physical properties
Hardness: 3.0-3.5 (Mohs); Color: Gray to light brown, revealing a distinct hexagonal pattern when wet; Luster: Dull to pearly; Crystal structure: Orthorhombic (as Calcite substitution); Specific Gravity: 2.7.
Formation & geological history
Formed through the fossilization of colonial rugose coral during the Devonian period, approximately 350 million years ago. These organisms lived in warm shallow seas that once covered Michigan.
Uses & applications
Primarily used as a decorative gemstone, in jewelry (pendants and rings), as a collector's specimen, and as a popular souvenir for tourists.
Geological facts
The Petoskey stone is the state stone of Michigan. The unique 'eye' pattern is actually the skeletal structure of individual corals that lived in a colony. They are often found rounded and smooth due to glacial movement and wave action.
Field identification & locations
Identify in the field by looking for the six-sided 'honeycomb' pattern on light gray stones, especially along lake shores. The pattern becomes much more visible when the stone is wet. Commonly found on the shores of Lake Michigan and Lake Huron.
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