sedimentary
Petoskey Stone
Hexagonaria percarinata (Fossilized Rugose Coral)
Hardness: 3-4 (Mohs), Color: Gray to light tan, Luster: Dull or waxy unless polished, Crystal Structure: Hexagonal patterns in the calcitic skeleton, Cleavage: None (fossilized structure)
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Physical properties
Hardness: 3-4 (Mohs), Color: Gray to light tan, Luster: Dull or waxy unless polished, Crystal Structure: Hexagonal patterns in the calcitic skeleton, Cleavage: None (fossilized structure)
Formation & geological history
Formed from ancient coral reefs during the Devonian Period (approx. 350 million years ago) in a warm, shallow sea that covered Michigan. The coral was later fossilized as silica replaced the calcium carbonate.
Uses & applications
State stone of Michigan, used frequently in jewelry (pendants, beads), lapidary art, and as decorative display specimens.
Geological facts
The distinctive 'eye' pattern is actually the cross-section of individual coral polyps. The name 'Petoskey' comes from an Ottawa Indian Chief, Pe-to-se-ga, meaning 'rays of the living sun'.
Field identification & locations
Identify in the field by looking for the hexagonal 'honeycomb' pattern that becomes visible when the stone is wet. It is most commonly found on the shores of Lake Michigan and Lake Huron.
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