sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3.0-3.5 on Mohs scale. Color: Grayish-brown when dry, reveals distinctive hexagonal pattern when wet. Luster: Dull to vitreous. Structure: Massive, containing fossilized coral polyps. Specific Gravity: 2.7.

Hardness: 3
Luster: Dull to vitreous

Identified March 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350-400 million years ago) from ancient warm saltwater seas. The prehistoric coral colonies were later covered by sediment and fossilized by mineral replacement (calcite).

Uses & applications

Commonly used in lapidary work for jewelry (cabochons), paperweights, and ornamental carvings. Highly valued as a state stone and for personal geological collections.

Geological facts

The Petoskey Stone is the state stone of Michigan. The pattern represents the skeleton of a fossilized colonial coral that lived in ancient seas long before the age of dinosaurs.

Field identification & locations

Identify by looking for a unique six-sided 'honeycomb' or starburst pattern. Best found on the shores of Lake Michigan and Lake Huron, especially after a storm when waves wash them up. They are easiest to spot when wet.