fossil

Petoskey Stone

Hexagonaria percarinata (Silicified/Calcitic)

AI-generated identification · may be incorrect

Hardness: 3.0-3.5 (Calcite) or 6.5-7.0 (if silicified); Color: Tan, brown, gray; Luster: Waxy to dull (natural) or glassy (polished); Structure: Hexagonal colonial coral pattern.

Hardness: 3
Color: Tan, brown, gray
Luster: Waxy to dull (natural) or glassy (polished)

Identified July 14, 2026More fossil →

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Physical properties

Hardness: 3.0-3.5 (Calcite) or 6.5-7.0 (if silicified); Color: Tan, brown, gray; Luster: Waxy to dull (natural) or glassy (polished); Structure: Hexagonal colonial coral pattern.

Formation & geological history

Formed during the Devonian Period (approx. 350-400 million years ago) from colonial rugose corals in shallow, warm seas that once covered Michigan.

Uses & applications

Jewelry, lapidary art, paperweights, souvenirs, and scientific study of prehistoric reefs.

Geological facts

It is the official state stone of Michigan. The name comes from Ottawa Chief Pet-O-Sega. The 'eye' patterns are the individual corallites of the fossilized coral colonial organism.

Field identification & locations

Identified by the distinct six-sided star/honeycomb pattern that becomes highly visible when wet or polished. Commonly found on beaches of Lake Michigan and Lake Huron.