fossil

Ammonite Fossil

Ammonoidea

Hardness typically 3 to 4 on the Mohs scale depending on mineral replacement; color ranges from dark brown, black, to grey with calcite or aragonite chamber infills; luster is vitreous to dull; spiral shell structure with internal chambers (septae); specific gravity around 2.6 to 2.7.

Identified August 4, 2026More fossil →

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Physical properties

Formation & geological history

Formed from the fossilized remains of extinct marine mollusk cephalopods that lived from the Devonian period to the Cretaceous-Paleogene extinction event (roughly 400 to 66 million years ago). Shells were buried in marine sediments and later mineralized through permineralization, often replaced by calcite, pyrite, or silica.

Uses & applications

Used widely in fossil collecting, lapidary arts, jewelry making, interior decoration, and as important index fossils for biostratigraphy.

Geological facts

Ammonites are more closely related to modern coleoids (octopuses, squid, cuttlefish) than to nautiluses, despite their external shell similarities. Some specimens can grow to be over two meters in diameter.

Field identification & locations

Easily identified by their characteristic planispiral shell shape and visible intricate suture lines on the surface. Commonly found in marine sedimentary rock layers worldwide, particularly in locations like Madagascar, Morocco, and parts of Europe and North America.