fossil

Fossil Coral

Hexagonaria (Rugose coral) / Rugosa

Hardness of 6.5 to 7 on the Mohs scale (due to silicification), colors typically ranging from brown, tan, gray to reddish-brown, vitreous to waxy luster, microcrystalline quartz (chalcedony/agate) replacement structure.

Identified August 3, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian period (approximately 350 to 400 million years ago) when warm, shallow seas covered the region. Coral skeletons were buried in sediment and preserved through petrifaction or silicification, where minerals replaced the original coral structure.

Uses & applications

Used primarily in lapidary arts, making cabochons for jewelry, decorative polished specimens, and prized by fossil and rock collectors.

Geological facts

Fossil coral is the state stone of Michigan, specifically the Petoskey stone variety, which features characteristic hexagonal coral patterns.

Field identification & locations

Easily identified by the distinctive honeycomb or web-like cellular patterns on the surface representing individual coral corallites. Commonly found in areas with ancient limestone and marine sedimentary deposits, particularly in the Midwestern United States.