fossil

Petoskey Stone / Fossilized Coral

Hexagonaria percarinata

Hardness: 3 to 4 on the Mohs scale; Color: Brown, gray, and tan with distinct hexagonal patterns; Luster: Dull to waxy when polished; Crystal structure: Trigonal (as calcite); Specific gravity: 2.6 to 2.8

Hardness: 3 to 4 on the Mohs scale
Color: Brown, gray, and tan with distinct hexagonal patterns
Luster: Dull to waxy when 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 an extinct rugose coral.

Uses & applications

Commonly used in lapidary arts, jewelry making, decorative items, and as a popular collectible specimen.

Geological facts

The Petoskey stone is the official state stone of Michigan. When wet or polished, the unique cellular coral pattern becomes dramatically more vibrant and detailed.

Field identification & locations

Easily identified by the characteristic pattern of six-sided corallites (cells) visible on the surface. Commonly found along the shores of Lake Michigan and Lake Huron, as well as glacial drift deposits in the northern US.

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