fossil

Petoskey Stone

Hexagonaria percarinata

Hardness of 3 to 4 on the Mohs scale, colors range from grey to brown when dry and darken when wet, vitreous to dull luster, hexagonal corallite structure with distinct eye-like centers, specific gravity of approximately 2.6 to 2.7.

Identified August 2, 2026More fossil →

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Physical properties

Formation & geological history

Formed through the fossilization of colonial rugose corals that lived in shallow warm seas during the Devonian period, approximately 350 million years ago, subsequently shaped by glacial activity in the Great Lakes region.

Uses & applications

Used primarily in lapidary arts, making polished stones, jewelry, decorative ornaments, and highly sought after by rockhounds and fossil collectors.

Geological facts

The Petoskey stone is the official state stone of Michigan. The unique hexagonal pattern represents individual coral polyps that once made up the ancient reef colony.

Field identification & locations

Easily identified by its characteristic pattern of six-sided coral cells. Most commonly found along the shores of Lake Michigan and Lake Huron, especially after being tumbled and smoothed by wave action.