fossil

Fossil coral fragment

Likely fossilized colonial coral, commonly Hexagonaria (Petoskey stone) or related tabulate coral

AI-generated identification · may be incorrect

Cream-gray, dense, and finely patterned with polygonal corallites; typically calcite, Mohs hardness 3, vitreous to dull luster, no obvious cleavage, and specific gravity about 2.6–2.8. The honeycomb-like cellular pattern is diagnostic of fossil coral.

Identified September 8, 2026More fossil →

Explore Fossil coral fragment 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

A Devonian marine coral colony was buried and its aragonitic skeleton replaced or recrystallized by calcite over roughly 350–420 million years. Such fossils formed in shallow tropical seas and were later exposed by erosion.

Uses & applications

Used as a collectible fossil, lapidary material, paperweight, and decorative stone; polished examples are sometimes sold as Petoskey stone. It is not generally used structurally.

Geological facts

The visible pattern represents individual coral polyp chambers, not mineral crystals. Petoskey stone is Michigan’s state stone, though similar fossil corals occur worldwide.

Field identification & locations

Identify it by repeated polygonal or star-like corallite cells across the surface; a wet or polished surface usually reveals the pattern more clearly. Common in Devonian limestone deposits around the Great Lakes and other ancient shallow-marine formations.