Rock Identifier
Iridescent Ammonite Fossil (Ammonoidea (subclass); Composition: Aragonite (Calcium Carbonate, CaCO3) often replaced by Calcite or Pyrite) — sedimentary
sedimentary

Iridescent Ammonite Fossil

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

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 More sedimentary →
Explore Iridescent Ammonite Fossil in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App StoreGoogle Play

Physical properties

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).

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.

More like this

Other sedimentary specimens

Sandstone or Siltstone (Arenite (if sandstone)) — Sedimentary

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary
Cone-in-cone structure (Limestone) (Cone-in-cone structure (Secondary Sedimentary Structure)) — sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary
Chert Breccia (Brecciated Chert (Microcrystalline Silica)) — sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary
Sandstone Grain (Clastic Sedimentary Rock Grain (SiO2 dominated)) — sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary
Shale or Slaty Mudstone (Argillaceous sedimentary rock) — sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary
Sandstone (Arenite (primarily SiO2)) — sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary