sedimentary
Petoskey Stone (Fossilized Rugose Coral)
Hexagonaria percarinata
Hardness: 3-4 (Mohs scale), Color: tan to gray, Luster: dull to vitreous, Structure: hexagonal patterns of the coral polyps, Cleavage: none (conchoidal fracture when silicified)
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Physical properties
Hardness: 3-4 (Mohs scale), Color: tan to gray, Luster: dull to vitreous, Structure: hexagonal patterns of the coral polyps, Cleavage: none (conchoidal fracture when silicified)
Formation & geological history
Formed during the Devonian Period about 350 million years ago when Michigan was covered by a warm, shallow sea. These are fossilized rugose corals that were later glaciated and deposited throughout the Great Lakes region.
Uses & applications
Commonly used for decorative lapidary work, jewelry (pendants and beads), and as a popular collector's item; it is the state stone of Michigan.
Geological facts
The unique 'eye' pattern is the skeletal structure of ancient coral. The name 'Petoskey' originates from an Ottawa Indian chief, Chief Pet-O-Sega.
Field identification & locations
Identified by the distinct six-sided star/hexagon pattern that becomes very prominent when the stone is wet. Commonly found on beaches of Lake Michigan and Lake Huron.
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