sedimentary

Fossilized Horn Coral

Order Rugosa (Rugose Coral)

AI-generated identification · may be incorrect

Hardness: 3-4 (Mohs scale), Color: tan, grey, or brownish-yellow, Luster: dull to earthy, Crystal Structure: often replaced by calcite (trigonal) or silica (hexagonal), notable for distinct radiating septa (lines) within a horn-shaped cup.

Identified March 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the fossilization of marine organisms in shallow tropical seas during the Paleozoic era (approx. 488 to 251 million years ago). The organic matter was replaced by minerals over geological time.

Uses & applications

Educational specimens for geology students, decorative items for rock collections, and occasionally used in lapidary work for unique jewelry cabochons.

Geological facts

Rugose corals are extinct. They earned the nickname 'horn coral' due to their resemblance to a bull's horn. Unlike modern corals, many were solitary rather than colonial.

Field identification & locations

Identify in the field by looking for a horn/cone shape with internal partitions (septa) radiating from the center. Commonly found in limestone outcrops in the Midwest USA and parts of the UK. Collectors look for well-preserved septa and lack of erosion.