sedimentary
Petoskey Stone
Hexagonaria percarinata
Hardness: 3-4 (Mohs scale), Color: tan to gray with distinctive dark-patterned hexagonal eyes, Luster: dull to vitreous when polished, Crystal Structure: hexagonal coral skeletons (calcite)
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Physical properties
Hardness: 3-4 (Mohs scale), Color: tan to gray with distinctive dark-patterned hexagonal eyes, Luster: dull to vitreous when polished, Crystal Structure: hexagonal coral skeletons (calcite)
Formation & geological history
Fossilized rugose coral from the Devonian period (approx. 350 million years ago), formed by the replacement of coral structure by calcite in a warm, shallow sea environment.
Uses & applications
State stone of Michigan; used extensively in jewelry making, lapidary art, paperweights, and as a popular collector's item.
Geological facts
The 'eyes' are actually the fossilized skeletons of individual coral polyps. The name comes from Ottawa Chief Pet-O-Sega, and the stones are exclusively found in the Alpena Limestone formation.
Field identification & locations
Identified by the unique six-sided hexagonal pattern (corallites) that becomes much more visible when the stone is wet. Commonly found along the shores of Lake Michigan and Lake Huron.
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