sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale), Color: Gray to brown when dry, pattern visible when wet. Luster: Dull to earthy (unpolished) or vitreous (polished). Structure: Hexagonal colonial coral patterns. Specific Gravity: 2.7.

Color: Gray to brown when dry, pattern visible when wet
Luster: Dull to earthy (unpolished) or vitreous (polished)

Identified May 31, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from fossilized colonial rugose corals from the Devonian period, approximately 350-400 million years ago. These corals lived in warm, shallow seas that once covered Michigan.

Uses & applications

Predominantly used for jewelry, decorative carvings, and as a popular collector's item. In the past, it was used for local construction/fill before its significance was realized.

Geological facts

It is the official state stone of Michigan. The name 'Petoskey' comes from an Ottawa Indian Chief, Pet-O-Sega. The 'eyes' in the stone are actually the individual coral polyps (corallites).

Field identification & locations

Identify by wetting the stone to reveal the unique hexagonal 'honeycomb' pattern. Commonly found along the shores of Lake Michigan and Lake Huron, particularly in the Petoskey area.