Iridescent ammonite is a rare, fossilized shell that displays vibrant, spectral play-of-color due to microscopic layers of aragonite. Collectors prize high-quality specimens for their intense, flashing rainbows, which are best observed when the surface is properly stabilized and polished. It is primarily sourced from the Bearpaw Formation in Alberta, Canada, where specific pressure and mineral conditions created this unique gemstone.
Is this iridescent ammonite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch iridescent ammonite with a known reference. Iridescent Ammonite sits at Mohs 3.5-4, softer than the next harder reference, harder than the previous one.
- 2Name the organismDecide what left the fossil before worrying about the rock. Organism: Ammonites Placenticeras meeki and P. intercalare. Age: Late Cretaceous (Bearpaw Formation).
- 3Check the preservationPreservation: Original aragonite shell, iridescent. That controls the weight, the hardness and how the piece takes a polish.
- 4Read the lusterHold the specimen under a strong light. Luster: pearly.
- 5Match the color rangeCompare against the expected color range: iridescent, red, green, gold, blue, purple.
Often confused with
Iridescent Ammonite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Labradorite is the harder of the two (Mohs 6-6.5 vs. 3.5-4); luster reads pearly on Iridescent Ammonite and vitreous; pearly on cleavages on Labradorite.
How to tell apart: Opal is the harder of the two (Mohs 5.5-6 vs. 3.5-4); luster reads pearly on Iridescent Ammonite and vitreous to dull on Opal.
Often found alongside iridescent ammonite
Minerals reported to co-occur with iridescent ammonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Mohs hardness
- 3.5-4
- Density
- 2.6-2.8 g/cm³
- Luster
- Pearly
- Transparency
- Opaque
- Rarity
- Rare
- Uses
- Gemstone, Collector, Jewelry
- Host rock
- Marine sedimentary shale
- Organism
- Ammonites Placenticeras meeki and P. intercalare
- Age
- Late Cretaceous (Bearpaw Formation)
- Preservation
- Original aragonite shell, iridescent
Where rockhounds find iridescent ammonite
Classic worldwide localities
- Alberta, Canada
- Madagascar
- United States
Field-hunting tip
Where it forms: Marine sedimentary shale. If you start seeing calcite, pyrite, siderite in float, you are in the right ground.
Common questions
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Find iridescent ammonite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
