sedimentary
Petoskey Stone (Fossilized Coral)
Hexagonaria percarinata
Hardness: 3-4 (Mohs), Color: Gray to tan (often appears reddish or orange if wet or stained by minerals), Luster: Dull to vitreous, Crystal Structure: Hexagonal patterns (corallites), Specific Gravity: 2.7
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Physical properties
Hardness: 3-4 (Mohs), Color: Gray to tan (often appears reddish or orange if wet or stained by minerals), Luster: Dull to vitreous, Crystal Structure: Hexagonal patterns (corallites), Specific Gravity: 2.7
Formation & geological history
Formed from fossilized rugose corals (Hexagonaria) that lived in warm Devonian seas approximately 350 to 400 million years ago. These colonies were buried in sediment and slowly mineralized into limestone.
Uses & applications
Used primarily in jewelry-making (beads, pendants), decorative stone carving, and as the official state stone of Michigan for local collecting.
Geological facts
The name comes from the Ottawa Chief Petosegay. The stone is unique because the hexagonal pattern is only visible when the stone is wet or polished; otherwise, it looks like an ordinary gray rock.
Field identification & locations
Can be identified by the 'spiderweb' or hexagonal pattern across the surface. Commonly found along the shores of Lake Michigan and Lake Huron, especially after storms turn over the lakebed.
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