fossil

Ammonite Fossil

Ammonoidea

Hardness: 3.5 to 4 on the Mohs scale (for calcite/aragonite replacements); Color: Dark brown, black, grey with white or iridescent chambers; Luster: Vitreous to pearly when polished; Crystal structure: Trigonal (calcite); Specific gravity: 2.6 to 2.7

Hardness: 3
Color: Dark brown, black, grey with white or iridescent chambers
Luster: Vitreous to pearly when polished

Identified August 4, 2026More fossil →

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Physical properties

Formation & geological history

Formed from the fossilized shell of marine mollusk cephalopods that lived from the Devonian period to the Cretaceous period (approximately 400 to 66 million years ago). The original shell material is often replaced by minerals like calcite or pyrite during fossilization in sedimentary rock layers.

Uses & applications

Used primarily in jewelry, decorative carvings, home decor, and as prized specimens for fossil and mineral collections.

Geological facts

Ammonites are index fossils, meaning geologists use them to determine the relative age of the rock strata in which they are found. They are closely related to modern-day nautiluses, squids, and octopuses rather than snails.

Field identification & locations

Easily identified by their characteristic spiral shape and chambered internal structure (septum lines). Commonly found in marine sedimentary rock formations worldwide, particularly in Europe, Madagascar, and North America.