fossil

Polished fossil coral

Fossilized colonial coral, commonly Hexagonaria (Petoskey stone) or related rugose/tabulate coral

AI-generated identification · may be incorrect

A carbonate fossil with pale coral-polyp patterns in a dark gray-brown matrix; calcite/aragonite replaced by calcite, Mohs hardness about 3–4, dull to waxy luster, and no consistent cleavage visible. The rounded or cut surface appears polished.

Identified September 4, 2026More fossil →

Explore Polished fossil coral in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Marine colonial coral was buried in carbonate sediment and mineralized during diagenesis, commonly in Paleozoic rocks roughly 250–450 million years old. Similar material occurs in shallow ancient-sea deposits.

Uses & applications

Collected as a fossil and used for cabochons, ornaments, and educational specimens; it is not generally used as structural stone. Value depends on pattern, polish, size, and locality.

Geological facts

The circular markings are individual coral corallites, not mineral crystals. Petoskey stone is the best-known example, but this specimen’s dark matrix and indistinct pattern prevent a precise genus or locality identification.

Field identification & locations

Identify it by repeated corallite/polyp motifs throughout the stone, unlike ordinary veined limestone; a dilute acid may fizz, though testing can damage polish. Common sources include Michigan, Iowa, Morocco, and parts of Europe.