sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Coral)

Hardness: 3-3.5 (Mohs scale), Color: Gray/brown when dry, reveals honeycomb pattern when wet; Luster: Dull to vitreous (when polished); Crystal Structure: Cryptocrystalline calcite replacements; Cleavage: None (fossilized structure).

Hardness: 3-3
Color: Gray/brown when dry, reveals honeycomb pattern when wet
Luster: Dull to vitreous (when polished)

Identified May 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period (approx. 350 million years ago) through the fossilization of colonial rugose coral. The coral colonies were later moved by glacial action during the Pleistocene epoch.

Uses & applications

Used primarily for jewelry (cabochons, pendants), decorative ornaments, and as a popular collector's item and souvenir.

Geological facts

It is the state rock of Michigan. The name comes from Ottawa Chief Pet-O-Sega. The characteristic 'eyes' are actually the individual coral polyps (corallites) that made up the colony.

Field identification & locations

Identify in the field by looking for the distinct six-sided 'honeycomb' pattern on rounded, gray beach stones, especially along the shores of Lake Michigan and Lake Huron. The pattern becomes highly visible when the stone is wet.

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