sedimentary

Iridescent Ammonite Fossil

Ammonoidea (subclass); Composition: Aragonite (Calcium Carbonate, CaCO3) often replaced by Calcite or Pyrite

AI-generated identification · may be incorrect

Hardness: 3.5-4.0 (Mohs scale); Color: Brownish-grey with rainbow iridescence (purple, green, orange); Luster: Pearly to iridescent (ammolite-like), dull where unpolished; Crystal Structure: Orthorhombic (aragonite shell); Cleavage: None (conchoidal fracture).

Hardness: 3
Luster: Pearly to iridescent (ammolite-like), dull where unpolished

Identified March 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through permineralization of extinct marine cephalopod shells in sedimentary basins during the Jurassic or Cretaceous periods (approx. 66–200 million years ago). The iridescent layer is caused by the preservation of the original nacreous (mother-of-pearl) layers of the shell.

Uses & applications

Used primarily as decorative collector specimens, educational fossils, and in the jewelry industry (high-quality iridescent segments are sometimes sold as 'Ammolite' gems).

Geological facts

Ammonites are more closely related to living octopuses and squids than to the Chambered Nautilus they resemble. The iridescence is a structural color caused by light interference as it passes through thin, stacked layers of aragonite platelets.

Field identification & locations

Identify in the field by the planispiral (flat-coiled) shell shape and distinctive 'suture' patterns on the surface. Commonly found in Madagascar, Morocco, and the Alberta Bearpaw Formation. Collectors look for unbroken spirals and vibrant color flashes across the surface.