fossil

Petoskey Stone

Hexagonaria percarinata

Mohs hardness: 3 to 4. Color: Grey, brown, or tan when dry, dark grey to black hexagonal patterns with white centers when wet. Luster: Dull to vitreous when polished. Crystal structure: Microcrystalline quartz (silicified coral skeleton). Specific gravity: 2.6 to 2.7.

Hardness: 3 to 4
Luster: Dull to vitreous when polished

Identified July 31, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian period, approximately 350 million years ago, when Michigan was located near the equator and covered by a warm, shallow sea. Composed of fossilized rugose coral.

Uses & applications

Popular as a lapidary material, carved into decorative objects, and used in jewelry such as pendants and rings. Highly collectible for fossil enthusiasts.

Geological facts

The Petoskey stone is the state stone of Michigan. The unique hexagonal pattern represents individual coral corallites that formed the colonial coral.

Field identification & locations

Easily identified by the distinctive hexagonal 'sunburst' or honeycomb pattern visible on the surface, especially when wet or polished. Found primarily in the glacial drift of northern Michigan.