fossil

Petoskey Stone

Hexagonaria percarinata

Hardness of 3 to 4 on the Mohs scale, colors range from gray to brown with distinct honeycomb or eye-like patterns, dull to waxy luster when polished, calcite composition, hexagonal corallites

Identified August 2, 2026More fossil →

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Physical properties

Hardness of 3 to 4 on the Mohs scale, colors range from gray to brown with distinct honeycomb or eye-like patterns, dull to waxy luster when polished, calcite composition, hexagonal corallites

Formation & geological history

Formed during the Devonian period approximately 350 million years ago when Michigan was covered by a warm, shallow tropical sea, created through the fossilization of colonial rugose corals

Uses & applications

Used primarily in lapidary arts, making decorative objects, jewelry, and as a popular state stone souvenir

Geological facts

It is the official state stone of Michigan, named after an Ottawa Native American chief named Pet-O-Sega

Field identification & locations

Easily identified by its characteristic pattern of six-sided coral cells which become much more prominent when wet or polished; commonly found along the shores of Lake Michigan and Lake Huron