sedimentary (fossilized)

Iridescent Ammonite

Ammonoidea (Species: likely Cleoniceras or Desmoceras); Mineral: Aragonite/Ammolite (CaCO3)

Hardness: 3.5-4.0 (Mohs scale); Color: Brown/Tan with opal-like iridescence (rainbow flash); Luster: Pearly to Vitreous; Structure: Planispiral coiled shell with complex suture patterns; Specific Gravity: 2.6-2.85.

Hardness: 3
Color: Brown/Tan with opal-like iridescence (rainbow flash)
Luster: Pearly to Vitreous

Identified March 11, 2026More sedimentary (fossilized) →

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Physical properties

Formation & geological history

Formed through the fossilization of marine cephalopods during the Cretaceous period (approx. 100 million years ago) specifically in Madagascar. The iridescence is preserved nacre (mother-of-pearl) within sedimentary shale or limestone layers.

Uses & applications

Predominantly used as decorative display pieces, in jewelry (cut as cabochons or set whole), and as a popular specimen for fossil collectors and classroom education.

Geological facts

The rainbow effect is caused by light interference as it passes through thin layers of aragonite crystals. Ammonites are related to the modern nautilus and went extinct at the same time as the dinosaurs approximately 66 million years ago.

Field identification & locations

Identified in the field by their distinct spiral shape and the 'ribbed' texture of the shell. They are common in the Mahajanga province of Madagascar. Collectors look for unbroken spirals and high-intensity 'fire' or color flashes.

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