fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3 to 4 on the Mohs scale. Color: Brownish-gray when dry, dark brown to black when wet or polished. Luster: Dull to greasy (vitreous when polished). Structure: Composed of fossilized rugose coral with distinct hexagonal corallites. Cleavage: None. Specific gravity: 2.6 to 2.7.

Hardness: 3 to 4 on the Mohs scale
Luster: Dull to greasy (vitreous when polished)

Identified August 4, 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 coral skeletons were preserved and fossilized through mineral replacement by calcite and silica.

Uses & applications

Commonly used in lapidary arts, jewelry making (cabochons, pendants), and as decorative polished display pieces or souvenirs.

Geological facts

The Petoskey stone is the official state stone of Michigan. The unique hexagonal pattern reveals the coral's corallites, which show star-like shapes when the stone is wet or polished.

Field identification & locations

Easily identified by its characteristic mottled gray-brown color and the distinct hexagonal patterns on the surface. Commonly found along the shores of Lake Michigan and Lake Huron, particularly near the Michigan Lower Peninsula.