sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (Mohs scale), Color: Gray/brown background with hexagonal internal patterns, Luster: Dull to vitreous when polished, Crystal structure: Hexagonal corallite cells, Cleavage: None/fossilized matrix, Specific Gravity: 2.7
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Gray/brown background with hexagonal internal patterns, Luster: Dull to vitreous when polished, Crystal structure: Hexagonal corallite cells, Cleavage: None/fossilized matrix, Specific Gravity: 2.7
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) from the fossilization of colonial rugose corals in warm, shallow saltwater seas. The area later became dry land (Michigan Basin).
Uses & applications
Used primarily for jewelry (beads, cabochons), home decor, tourist souvenirs, and fossil collecting.
Geological facts
This is Michigan's state stone. The name 'Petoskey' comes from an Ottawa Indian Chief, Pe-to-ga, meaning 'Rising Sun'. The pattern is only clearly visible when the stone is wet or polished.
Field identification & locations
Identify by looking for the 6-sided 'honeycomb' pattern. It is commonly found on the beaches of northern Lake Michigan and Lake Huron. Collectors find them by walking along the shoreline after a storm.
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