sedimentary

Petoskey Stone (Fossilized Rugose Coral)

Hexagonaria percarinata (Calcium Carbonate, CaCO3)

Hardness: 3 - 3.5 on Mohs scale; Color: Gray to beige (becomes distinctive pattern when wet); Luster: Dull to vitrous (when polished); Crystal Structure: Orthorhombic/Hexagonal (fossil structure); Cleavage: None; Specific Gravity: Approx 2.7

Hardness: 3 - 3
Color: Gray to beige (becomes distinctive pattern when wet)
Luster: Dull to vitrous (when polished)

Identified April 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from fossilized rugose coral colonies that lived in warm Devonian seas approximately 350 million years ago. Glaciation later plucked these fossils from the bedrock and deposited them as glacial drift.

Uses & applications

Used primarily for jewelry (cabochons), paperweights, decorative carvings, and as a popular collector's item and state stone of Michigan.

Geological facts

Petoskey stones are unique to the Great Lakes region. The name 'Petoskey' comes from an Ottawa Indian chief, Ignatius Petosega. When dry, it looks like ordinary limestone, but when wet or polished, the unique 'six-sided' coral pattern emerges.

Field identification & locations

Identified in the field by looking for the 'honeycomb' or 'star' patterns on gray, rounded stones found on beaches or in gravel pits. Most common along the shores of Lake Michigan and Lake Huron.

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