sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-3.5 (Mohs scale), Color: Brownish-tan to gray with hexagonal pattern, Luster: Vitreous to dull, Crystal Structure: Hexagonal (pattern), Cleavage: None/unobservable, Specific Gravity: 2.7
- Hardness
- 3-3
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Physical properties
Hardness: 3-3.5 (Mohs scale), Color: Brownish-tan to gray with hexagonal pattern, Luster: Vitreous to dull, Crystal Structure: Hexagonal (pattern), Cleavage: None/unobservable, Specific Gravity: 2.7
Formation & geological history
Formed during the Devonian Period (approx. 350 million years ago) as coral reefs in the warm, shallow salt water of what is now Michigan. Over time, the living coral was replaced by calcite (limestone).
Uses & applications
Used primarily for jewelry, lapidary art, paperweights, and as a popular collector's item and state stone of Michigan.
Geological facts
It is Michigan's official State Stone. The name 'Petoskey' comes from an Ottawa Indian chief, Chief Pet-O-Sega. The 'eye' pattern is the fossilized structure of individual coral polyps.
Field identification & locations
Identify by a distinct 'honeycomb' or hexagonal pattern that becomes prominent when wet. Found on beaches along the northern shores of Michigan's lower peninsula and in gravel pits.
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