fossil

Fossilized Coral (Petoskey Stone / Hexagonaria)

Hexagonaria percarinata / Rugosa coral (CaCO3)

Hardness 3-4 on Mohs scale, grayish-brown matrix with lighter circular or polygonal eyes, dull to vitreous luster, fine microcrystalline texture, effervesces in warm acid.

Identified July 23, 2026More fossil →

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Physical properties

Hardness 3-4 on Mohs scale, grayish-brown matrix with lighter circular or polygonal eyes, dull to vitreous luster, fine microcrystalline texture, effervesces in warm acid.

Formation & geological history

Formed during the Middle Devonian period (about 350 million years ago) when warm, shallow tropical seas covered North America. Formed by ancient rugose coral colonies preserved through mineralization and weathered by glaciers.

Uses & applications

Popular among rockhounds for collecting, polishing, lapidary arts, jewelry (cabochons, pendants), and decorative crafts.

Geological facts

Petoskey stones are the state stone of Michigan. The unique 'eye' pattern is formed by individual corallites that made up the colonial coral organism, Hexagonaria.

Field identification & locations

Look for distinct circular or hexagonal cell patterns with fine radiating lines inside. Commonly found along shorelines, gravel pits, and riverbeds in the Great Lakes region, especially northern Michigan.

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