sedimentary

Petoskey Stone

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3-3.5 on Mohs scale. Color: Gray, tan, or brown with distinct hexagonal patterns appearing as 'bursts' or stars. Luster: Dull to waxy when raw, vitreous to silky when polished. Composition: Primarily calcite (calcium carbonate).

Hardness: 3-3
Luster: Dull to waxy when raw, vitreous to silky when polished

Identified April 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from fossilized colonial rugose corals during the Devonian period, approximately 350+ million years ago. These corals lived in warm, shallow seas that once covered Michigan.

Uses & applications

Primarily used for jewelry making, lapidary art, paperweights, and as a popular souvenir/collectible.

Geological facts

It is the state stone of Michigan. The unique 'eye' pattern is the fossilized skeleton of individual coral polyps. When dry, the stone looks like ordinary limestone, but the pattern becomes vivid when wet or polished.

Field identification & locations

Most commonly found on the shores of Lake Michigan and Lake Huron. To identify in the field, look for smooth, gray stones and wet them with water to reveal the hexagonal honeycomb pattern. Collectors should search beaches after a storm for freshly unearthed pieces.

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