fossil

Fossil Coral

Rugose Coral / Hexagonaria (CaCO3)

Hardness of 3 on the Mohs scale, typically gray, tan, or brown with distinct star-like or honeycomb patterns, vitreous to dull luster, crystal structure is hexagonal/trigonal (calcite replacement).

Identified August 1, 2026More fossil →

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Physical properties

Hardness of 3 on the Mohs scale, typically gray, tan, or brown with distinct star-like or honeycomb patterns, vitreous to dull luster, crystal structure is hexagonal/trigonal (calcite replacement).

Formation & geological history

Formed in ancient warm, shallow marine environments during the Paleozoic era (millions of years ago) where coral skeletons were progressively replaced by silica or calcite through permineralization.

Uses & applications

Used primarily as decorative lapidary material, in jewelry making, and highly sought after by fossil and rock collectors.

Geological facts

Many fossil corals, particularly Petoskey stones, are glacial erratics and serve as the state stone of Michigan.

Field identification & locations

Easily identified in the field by the distinct branching or honeycomb corralite structures visible on the weathered surface. Commonly found in gravel beds, glacial tills, and limestone formations.