fossil

Fossil Coral (Petoskey / Horn Coral Sliced Button)

Scleractinia / Rugosa (Silurian-Devonian Anthozoa)

Hardness: 3-3.5 (calcite) to 6.5-7 (if silicified/agate-replaced). Color: tan, pinkish-brown, cream, grey. Luster: vitreous to dull. Structure: radial septa forming a star-like or wheel pattern in cross section. Cleavage: non-cleavable (fossil matrix). Specific Gravity: 2.6-2.8.

Hardness: 3-3
Color: tan, pinkish-brown, cream, grey
Luster: vitreous to dull

Identified July 23, 2026More fossil →

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Physical properties

Formation & geological history

Formed in warm, shallow marine waters during the Paleozoic era (Devonian to Carboniferous periods, around 350-400 million years ago). The organic skeleton of ancient coral polyps was buried in sediment and gradually replaced by calcite or silica over geological time.

Uses & applications

Popular among fossil collectors, used in wire-wrapping, cabochon crafting, lapidary jewelry, and educational display specimens.

Geological facts

Slice cross-sections of horn corals or solitary rugose corals display intricate internal septal patterns that look like flowers or stars. Small polished 'buttons' like this are often sold as 'fossil coral eyes' or sliced coral buttons.

Field identification & locations

Identified in the field by its distinct radial star/wheel structure showing septa extending from a central axis outward. Commonly found in ancient marine limestone beds in the Midwestern US (e.g., Michigan, Indiana, Ohio) and Morocco.

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