sedimentary
Petoskey Stone (Fossilized Rugose Coral)
Hexagonaria percarinata
Hardness: 3-4 (Mohs scale), Color: tan to gray with hexagonal patterns when wet/polished, Luster: dull to vitreous, Crystal Structure: hexagonal coral skeletons (calcite), Specific Gravity: 2.7
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Physical properties
Hardness: 3-4 (Mohs scale), Color: tan to gray with hexagonal patterns when wet/polished, Luster: dull to vitreous, Crystal Structure: hexagonal coral skeletons (calcite), Specific Gravity: 2.7
Formation & geological history
Formed during the Devonian Period (350+ million years ago) from colonial rugose corals in warm, shallow seas. The skeletons were buried and replaced by calcite through petrification.
Uses & applications
Commonly used in jewelry, lapidary arts, decorative items, and as a popular collectible specimen. Michigan's official state stone.
Geological facts
The name 'Petoskey' comes from an Ottawa Indian Chief, Pet-O-Sega. The patterns visible on the stone are the fossilized remains of coral polyps.
Field identification & locations
Identified by its distinctive six-sided 'honeycomb' pattern. Best found on the shores of Lake Michigan after waves have tumbled them; the pattern is most visible when the stone is wet.
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