sedimentary

Fossilized Coral (Petoskey-style)

Hexagonaria percarinata (Fossilized Rugose Coral)

Hardness: 3-4 (Calcite-based) or 6.5-7 (Silicified); Color: Light grey, white, buff; Luster: Dull to earthy (unpolished); Structure: Hexagonal or radiating corallites; Cleavage: None (structural fossil pattern)

Hardness: 3-4 (Calcite-based) or 6
Color: Light grey, white, buff
Luster: Dull to earthy (unpolished)

Identified May 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Devonian Period, approximately 350-400 million years ago, when warm prehistoric seas covered the land. The living coral skeletons were buried in sediment and replaced by calcite or silica over geological time.

Uses & applications

Used for decorative jewelry, paperweights, lapidary arts, and scientific study. High-quality specimens are popular collector pieces.

Geological facts

Petoskey stones are the state stone of Michigan. The pattern is formed by the fossilized individual polyps that made up the coral colony. When wet or polished, the intricate 'star' or 'honeycomb' pattern becomes much more visible.

Field identification & locations

Identified by the distinct honeycombed or radiating pattern of individual coral cells. Commonly found on lake shores (especially Lake Michigan) or in limestone quarries. In the field, look for a 'snake-skin' pattern on dry rocks that turns clear and patterned when wet.

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