sedimentary

Petoskey Stone

Hexagonaria percarinata

Hardness: 3-3.5 on Mohs scale; Color: Gray and beige with distinctive hexagonal patterns; Luster: Matte to waxy (pearly when wet or polished); Crystal Structure: Orthorhombic (fossilized calcite); Cleavage: Perfect rhombohedral; Specific Gravity: 2.7

Hardness: 3-3
Color: Gray and beige with distinctive hexagonal patterns
Luster: Matte to waxy (pearly when wet or polished)

Identified May 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by fossilization of ancient colonial rugose coral (Hexagonaria) during the Devonian period, roughly 350 million years ago, in shallow warm seas that once covered Michigan. Glaciation later spread these stones across the region.

Uses & applications

Used primarily in jewelry making (pendants, rings), decorative lapidary arts, and highly sought after by rocky hobbyists and fossil collectors.

Geological facts

It is the state stone of Michigan. The unique 'eye' pattern only appears clearly when the stone is wet or polished because the dry fossilized coral cells are often indistinguishable from the matrix.

Field identification & locations

Identify in the field by looking for the honeycomb or hexagonal pattern on rocks near shorelines, especially after rain or when wet. Most commonly found on the beaches of Lake Michigan and Lake Huron.