sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3.0-3.5 (Mohs scale); Color: Gray to tan when dry, distinct six-sided star patterns appear when wet; Luster: Dull to vitreous when polished; Structure: Calcite replacement of coral skeleton; Cleavage: None (fossilized structure).

Hardness: 3
Luster: Dull to vitreous when polished

Identified March 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) when Michigan was a warm, shallow tropical sea. The coral was later fossilized as calcite replaced the organic structures and then moved by glacial action during the Pleistocene.

Uses & applications

Used primarily in jewelry (beads, pendants), decorative ornaments, souvenirs, and as the official state stone of Michigan. It is highly valued by lapidary hobbyists for its ease of polishing.

Geological facts

It is not actually a rock but a fossil of a colonial coral. The name 'Petoskey' comes from an Ottawa Chief, Pe-to-se-ga, meaning 'rising sun' or 'rays of dawn,' referencing the sunburst pattern of the coral.

Field identification & locations

Identify in the field by looking for a 'brain-like' or hexagonal pattern on gray stones along shorelines, especially after rain or by wetting the stone. Predominantly found on the shores of Lake Michigan and Lake Huron in Michigan's Lower Peninsula.

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