fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3-3.5 on Mohs scale; Color: Gray, tan, or brown; Luster: Matte to vitreous when polished; Structure: Hexagonal coral cells (corallites) with radiating lines (septa); Specific Gravity: 2.7.

Hardness: 3-3
Color: Gray, tan, or brown
Luster: Matte to vitreous when polished

Identified July 14, 2026More fossil →

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Physical properties

Hardness: 3-3.5 on Mohs scale; Color: Gray, tan, or brown; Luster: Matte to vitreous when polished; Structure: Hexagonal coral cells (corallites) with radiating lines (septa); Specific Gravity: 2.7.

Formation & geological history

Formed from fossilized rugose coral from the Devonian period (approx. 350 million years ago) in shallow, warm seas that once covered Michigan. Glacial action later moved and polished these specimens.

Uses & applications

Used primarily in jewelry, lapidary art, paperweights, and as a popular souvenir/collector's item. It is the official state stone of Michigan.

Geological facts

The name comes from an Ottawa Chief named Petosegay. The 'sun ray' patterns are actually the skeletons of ancient coral colonies. They are most easily spotted when wet, as the pattern becomes vibrant and visible against the gray background.

Field identification & locations

Identify by looking for the hexagonal 'honeycomb' pattern on a gray pebble. Most commonly found on the shores of Lake Michigan and Lake Huron. Search specifically following storms when the waves have turned over the gravel on the beaches.