fossil

Petoskey Stone

Favosites alpenensis

Hardness: 3-4 on the Mohs scale; Color: Brownish-gray when dry, dark brown or black with white honeycomb patterns when wet; Luster: Waxy to dull when unpolished; Crystal structure: Hexagonal corallites (calcified coral skeletons)

Hardness: 3-4 on the Mohs scale
Luster: Waxy to dull when unpolished

Identified July 30, 2026More fossil →

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Physical properties

Formation & geological history

Formed from colonial rugose and tabulate corals that lived in warm, shallow seas covering Michigan during the Devonian period, approximately 350 million years ago.

Uses & applications

Used primarily in lapidary arts, decorative items, souvenirs, and jewelry-making.

Geological facts

The Petoskey stone is the state stone of Michigan. The distinct hexagonal pattern comes from the close-packed corallites that made up the coral colony, each featuring a star-like center.

Field identification & locations

Easily identified by its characteristic honeycomb or web-like pattern on the surface. They are most commonly found along the shores of Lake Michigan and Lake Huron, particularly after being smoothed by wave action. Collectors often search for them when wet, as the pattern becomes much more prominent.