sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 on Mohs scale; Color: Gray to tan when dry, distinct honeycomb pattern when wet; Luster: Dull to earthy (unpolished); Crystal Structure: Hexagonal skeletal chambers; Cleavage: None (fossilized structure).
- Hardness
- 3-4 on Mohs scale
- Color
- Gray to tan when dry, distinct honeycomb pattern when wet
- Luster
- Dull to earthy (unpolished)
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Physical properties
Hardness: 3-4 on Mohs scale; Color: Gray to tan when dry, distinct honeycomb pattern when wet; Luster: Dull to earthy (unpolished); Crystal Structure: Hexagonal skeletal chambers; Cleavage: None (fossilized structure).
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago). Originally soft-bodied coral living in warm, shallow seas; over time, the coral skeletal remains were buried by sediment and underwent permineralization, replacing calcite with silica and other minerals.
Uses & applications
Primarily used for lapidary work, jewelry (pendants and beads), home decor, and as a popular souvenir/collector item. In industry, the host limestone is used for aggregate, but specific Petoskey specimens are kept for aesthetic value.
Geological facts
It is the state stone of Michigan. The name 'Petoskey' is derived from a legendary Ottawa chief, Chief Pet-O-Sega, which means 'Rising Sun'. The stones were moved by glacial action during the last Ice Age and deposited across the Lower Peninsula.
Field identification & locations
Identify by looking for the 'honeycomb' or 'six-sided' star-like patterns on the surface. They are most easily found on the shores of Lake Michigan after a storm or when the stone is wet. Collectors should check the Little Traverse Bay area.
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