sedimentary
Fossilized Rugose Coral (Petoskey Stone type)
Hexagonaria percarinata (Order: Rugosa)
Hardness: 3-4 (Mohs); Color: Grey, tan, or white with distinct honeycomb/star patterns; Luster: Dull to pearly when raw, vitreous when polished; Structure: Calcite fossil replacements.
- Hardness
- 3-4 (Mohs)
- Color
- Grey, tan, or white with distinct honeycomb/star patterns
- Luster
- Dull to pearly when raw, vitreous when polished
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Physical properties
Hardness: 3-4 (Mohs); Color: Grey, tan, or white with distinct honeycomb/star patterns; Luster: Dull to pearly when raw, vitreous when polished; Structure: Calcite fossil replacements.
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) from the fossilized remains of ancient colonial coral reefs in warm, shallow saltwater seas.
Uses & applications
Used primarily for lapidary work, jewelry (pendants and beads), ornamental stones, and as a popular souvenir or collector's item in the Great Lakes region.
Geological facts
The Petoskey stone is the state stone of Michigan. The pattern seen on the surface results from the skeleton of the coral polyps, which contained living tissue during the Devonian age.
Field identification & locations
Look for the distinctive 'six-sided' or 'star' patterns on smooth, water-worn grey pebbles along shorelines. Wetting the stone often reveals the pattern hidden by the dry surface.
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