sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-3.5 on Mohs scale; Color: Gray to tan when dry, reveals hexagonal patterns when wet; Luster: Dull to vitreous (when polished); Crystal Structure: Hexagonal chambers; Cleavage: None (fossilized structure).
- Hardness
- 3-3
- Color
- Gray to tan when dry, reveals hexagonal patterns when wet
- Luster
- Dull to vitreous (when polished)
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Physical properties
Hardness: 3-3.5 on Mohs scale; Color: Gray to tan when dry, reveals hexagonal patterns when wet; Luster: Dull to vitreous (when polished); Crystal Structure: Hexagonal chambers; Cleavage: None (fossilized structure).
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) from ancient rugose coral reefs. Over time, the coral skeletons were replaced by calcite and buried in sedimentary limestone before being smoothed by glacial action.
Uses & applications
Used primarily in jewelry making (lapidary), as decorative paperweights/ornaments, and as highly sought-after souvenirs and collector's items.
Geological facts
It is the official state stone of Michigan, USA. The name 'Petoskey' comes from an Ottawa Indian Chief named Pet-O-Sega, meaning 'Rising Sun'. While they look like ordinary limestone when dry, their intricate 'eye' pattern appears vividly when the stone is wet.
Field identification & locations
Identify by looking for unique six-sided 'cells' with dark centers. Most commonly found along the shores of Lake Michigan and Lake Huron. Collectors find them most easily after a storm or a spring thaw when waves have turned up new stones.
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