fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 4 to 5 on the Mohs scale; Color: Brown, gray, or tan with distinctive six-sided coral corallite patterns; Luster: Waxy to dull when dry, shiny when wet or polished; Crystal structure: Microcrystalline quartz replacement of fossilized coral.

Hardness: 4 to 5 on the Mohs scale
Luster: Waxy to dull when dry, shiny when wet or polished

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian period, approximately 350 million years ago, when Michigan was covered by a warm, shallow tropical sea. The stones are fossilized colonies of ancient rugose coral.

Uses & applications

Used primarily in lapidary arts, making jewelry, decorative objects, and as a popular souvenir or collector's item in the Great Lakes region.

Geological facts

The Petoskey stone is the official state stone of Michigan. The name originates from Chief Petoskey of the Ottawa people, meaning 'rays of dawn' or 'sunbeams of promise'.

Field identification & locations

Easily identified by the characteristic pattern of hexagonal or honeycomb-like coral cells visible on the surface. Commonly found along the shores of Lake Michigan and Lake Huron, as well as in glacial till and gravel pits throughout northern Michigan.