fossil

Ammonite Fossil

Ammonoidea (subclass)

Hardness: 3.5 to 4 on Mohs scale; Color: Brown, tan, and cream; Luster: Pearly to dull; Structure: Planispiral coiled shell with visible internal chambers (septa); Composition: Often replaced by calcite or aragonite.

Hardness: 3
Color: Brown, tan, and cream
Luster: Pearly to dull

Identified July 18, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Devonian period through the Cretaceous (about 400 to 66 million years ago). They were marine cephalopods that fossilized in sedimentary marine environments after being buried in sediment that mineralized over millions of years.

Uses & applications

Primarily used for educational study, fossil collecting, and jewelry. Small polished halves are popular as pendants or cabinet specimens.

Geological facts

Ammonites are extinct relatives of the modern nautilus, squid, and octopus. They are 'index fossils,' meaning scientists use specific species to date the rock layers in which they are found. Highly iridescent specimens are known as 'ammolite' and are considered a gemstone.

Field identification & locations

Identify by their distinct spiral shape and chambered internal structure (seen when cut). Often found in limestone or shale cliffs, particularly in areas like Madagascar, Morocco, and the Jurassic Coast of England.