sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (patterns appear when wet), Luster: Dull to vitreous, Structure: Hexagonal colonial coral patterns, Cleavage: None
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry (patterns appear when wet), Luster: Dull to vitreous, Structure: Hexagonal colonial coral patterns, Cleavage: None
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) through the fossilization of colonial rugose corals in warm, shallow saltwater seas that once covered Michigan.
Uses & applications
Used primarily for jewelry (cabochons), pocket stones, decorative home decor, and as a popular collector's item for rockhounds.
Geological facts
It is the state stone of Michigan. The name comes from an Ottawa Chief, Pe-to-ga, meaning 'Rising Sun'. The unique 'eyes' are actually individual coral organisms called polyps.
Field identification & locations
Identify in the field by looking for a distinctive six-sided 'honeycomb' or 'star' pattern on stones along Great Lakes beaches (especially Lake Michigan). The pattern is clearest when the stone is wet.
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