fossil

Fossil Coral

Phylum Cnidaria / Rugosa or Tabulata

Identified July 29, 2026More fossil →

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Physical properties

Hardness varies around 3 to 4 on the Mohs scale depending on the replacing mineral (often calcite or silica which can raise it to 7); Color typically ranges from white, tan, and brown to reddish pink; Luster is dull to vitreous; Structure is often replaced by microcrystalline quartz (chalcedony) or calcite preserving the original coral skeletal septa.

Formation & geological history

Formed in warm, shallow marine environments millions of years ago (often Paleozoic or Cenozoic eras) where coral polyps secreted calcium carbonate skeletons that were subsequently fossilized and often permineralized or silicified over time.

Uses & applications

Used in lapidary arts, cabochons for jewelry, ornamental display, and as a key index fossil for geological studies.

Geological facts

Many fossil corals from ancient seas are the state gemstone or fossil of various regions (e.g., Petoskey stone in Michigan). As reefs grew and were buried by sediments, the organic material was replaced by minerals molecule by molecule, perfectly preserving the coral structure.

Field identification & locations

Can be identified in the field by distinctive honeycomb or star-like patterns (corallites) on the weathered surface. Commonly found in ancient limestone deposits, riverbeds, and beaches where sedimentary layers erode.