fossil

Ammonite Fossil

Ammonoidea

Hardness varies by mineral replacement (often Mohs 3.5-4 for calcite or 6.5-7 for quartz/pyrite), color ranges from dark grey, black, brown to iridescent tones, vitreous to dull luster, spiral external shell structure with intricate suture lines.

Identified August 1, 2026More fossil →

Explore Ammonite Fossil 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

Formed from marine mollusks that lived during the Paleozoic and Mesozoic eras (approximately 400 to 66 million years ago). When the organisms died, their shells settled on the sea floor, were buried in sediment, and gradually replaced by minerals over millions of years.

Uses & applications

Used in fossil collecting, education, decorative objects, lapidary art, and jewelry.

Geological facts

Ammonites are index fossils, meaning geologists use them to determine the relative ages of rock strata. They are closely related to modern octopuses, squid, and cuttlefish rather than nautiluses.

Field identification & locations

Found worldwide in marine sedimentary rocks, particularly in shales, limestones, and clay formations. Look for distinct coiled spiral shapes and complex wavy suture patterns on the surface.