sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Coral)
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (shows hexagonal pattern when wet), Luster: Dull to vitreous, Structure: Hexagonal colonial coral cells, Cleavage: None
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (shows hexagonal pattern when wet), Luster: Dull to vitreous, Structure: Hexagonal colonial coral cells, Cleavage: None
Formation & geological history
Formed through the fossilization of colonial rugose coral during the Devonian Period (approx. 350 million years ago) in warm, shallow seas. Glaciations later plucked these stones from the bedrock and spread them across Michigan.
Uses & applications
Highly valued as a state stone, used for biological study, lapidary arts (jewelry, cabochons, carvings), and as a popular collector's item.
Geological facts
It is the state stone of Michigan. The name 'Petoskey' comes from an Ottawa Indian Chief, Pet-O-Sega. The stone's patterns become clearly visible only when the stone is wet or polished.
Field identification & locations
Identify in the field by looking for 'six-sided cells' or sunburst patterns on gray stones, especially on the shores of Lake Michigan. Best found after a storm or in the spring when the ice thaws.
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