sedimentary
Petoskey Stone
Hexagonaria percarinata
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry, reveals hexagonal patterns when wet. Luster: Dull (natural) to Vitreous (polished). Structure: Hexagonal corallites. Cleavage: None. Specific Gravity: 2.6-2.7.
- Color
- Gray to tan when dry, reveals hexagonal patterns when wet
- Luster
- Dull (natural) to Vitreous (polished)
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Physical properties
Hardness: 3-4 (Mohs scale), Color: Gray to tan when dry, reveals hexagonal patterns when wet. Luster: Dull (natural) to Vitreous (polished). Structure: Hexagonal corallites. Cleavage: None. Specific Gravity: 2.6-2.7.
Formation & geological history
Formed during the Devonian Period about 350 million years ago. It is a fossilized rugose coral that lived in warm, shallow saltwater seas covering Michigan. Glacial action eventually spread the fossils across the Great Lakes region.
Uses & applications
Commonly used in jewelry, decorative items, as a popular state souvenir, and highly prized by rock collectors.
Geological facts
It is the official state stone of Michigan. The name comes from Ottawa Chief Pet-O-Sega, and the patterned 'eyes' are actually individual coral polyps.
Field identification & locations
Identify in the field by looking for the distinctive hexagonal pattern on gray cobbles along shorelines. Pattern is best seen when the stone is wet. Primarily found on the shores of Lake Michigan and Lake Huron.
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