fossil

Petoskey Stone

Hexagonaria percarinata

Hardness of 3 to 4 on the Mohs scale, grayish-white to brownish color when dry with a distinct honeycomb or reticulated pattern, waxy luster when polished, hexagonal coral corallites structure, specific gravity around 2.6-2.7.

Identified July 29, 2026More fossil →

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Physical properties

Formation & geological history

Formed from the fossilization of colonial rugose corals that lived in warm, shallow subtropical seas during the Devonian period, approximately 350 million years ago, particularly in the Michigan basin.

Uses & applications

Commonly used in lapidary arts for making polished pocket stones, jewelry items such as pendants and earrings, and as a popular souvenir for collectors.

Geological facts

The Petoskey stone is the official state stone of Michigan. The honeycomb pattern becomes dramatically more visible when the stone is wet or polished because water fills the microscopic pores in the calcite fossil skeleton, enhancing contrast.

Field identification & locations

Easily identified in the field along beaches and gravel pits in the upper Midwest of the USA by its characteristic mottled, six-sided coral pattern resembling a honeycomb or mosaic. Best found while beachcombing along the shores of Lake Michigan and Lake Huron.