fossil

Petoskey Stone

Hexagonaria percarinata

Hardness: 3-4 (Mohs scale), Color: Gray to tan with distinctive hexagonal honeycomb pattern, Luster: Dull/waxy (unpolished) to vitreous (polished), Crystal Structure: Orthorhombic (calcite-based replacement)

Identified July 6, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350-400 million years ago) from colonial rugose corals that lived in warm, shallow seas covering what is now Michigan. The skeletal structures were eventually replaced by minerals like calcite or chalcedony.

Uses & applications

State stone of Michigan; used frequently in lapidary work for jewelry, cabochons, decorative ornaments, and as a popular souvenir/collector's item.

Geological facts

The 'eyes' or honeycomb patterns seen in the stone are the fossilized individual coral polyps. The name comes from Ottawa Chief Pet-O-Sega, meaning 'Rising Sun' or 'Rays of Hope'.

Field identification & locations

Identified by the unique hexagonal cell pattern which becomes much more visible when wet or polished. Commonly found on the shores of Lake Michigan and Lake Huron, particularly near the Petoskey area.