Rock Identifier
Petoskey Stone (Hexagonaria percarinata) — fossil
fossil

Petoskey Stone

Hexagonaria percarinata

AI-generated identification · may be incorrect

Mohs hardness 3 to 4; gray to brown when dry, dark amber with distinct hexagonal coral patterns when wet; dull to earthy luster.

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Physical properties

Mohs hardness 3 to 4; gray to brown when dry, dark amber with distinct hexagonal coral patterns when wet; dull to earthy luster.

Formation & geological history

Formed from fossilized colonial rugose corals during the Devonian period, approximately 350 million years ago, in ancient shallow seas.

Uses & applications

Popular in lapidary arts, jewelry making, and as a regional collectible souvenir.

Geological facts

It is the official state stone of Michigan and features distinctive eye-like coral patterns that become prominent when polished or wet.

Field identification & locations

Found predominantly along the shores of Lake Michigan and northern lower Michigan; easily identified in the field by its characteristic mosaic of six-sided coral corallites.

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