Iridescence
Iridescence in gemstones including fire agate, ammolite, iris quartz, and surface coatings with causes and identification.
By gemmology.dev editors
Last updated
phenomena/iridescence fire-agate ammolite thin-film interference
Introduction
Iridescence is a display of spectral colours caused by light interference from
thin layers or structures near the surface. Unlike opal's play of colour
(diffraction from spheres), iridescence results from thin-film interference –
the same phenomenon that creates colours in soap bubbles and oil slicks.
Iridescence appears as rainbow sequences of colour that shift with viewing angle.
Mechanism
The physics of iridescence:
Thin-Film Interference
- Light reflects from both top and bottom of thin layers
- The two reflected waves interfere (add or cancel)
- Layer thickness determines which wavelengths reinforce
- Changing viewing angle changes apparent thickness → colour shifts
Requirements
- Thin layers (hundreds of nanometres thick)
- Different refractive indices between layers
- Transparent to semi-transparent layers
- Regular or semi-regular layer structure
Surface vs Internal Iridescence
Surface Iridescence
- Thin film on surface
- May be coating or tarnish
- Often from treatment
- Examples - coated topaz, "mystic" gems
Internal Iridescence
- Layers within the stone
- Natural formation
- More stable typically
- Examples - fire agate, ammolite
Fire Agate
Fire agate shows iridescent "flames" from thin iron oxide layers:
Formation
- Layers of iron oxide (goethite/limonite) deposited during growth
- Layers form over botryoidal (grape-like) chalcedony surface
- Creates three-dimensional colour patterns
- Each layer adds to interference effect
Characteristics
- Colours: Orange, red, green, gold, purple possible
- Pattern: Flame-like or cellular
- Structure: Botryoidal surface creates depth
- Source: Primarily Mexico and southwestern USA
Cutting Approach
- Carved rather than cabochon cut
- Must preserve botryoidal structure
- Remove matrix carefully to expose colours
- Skill required to maximise effect
Ammolite
Ammolite is fossilised ammonite shell from Alberta, Canada:
Formation
- Fossilised nacre (aragonite) from ammonite shells
- ~70 million years old (Cretaceous period)
- Thin nacre layers create interference
- Found only in Alberta's Bearpaw Formation
Characteristics
- Colours: Full spectrum possible; red-green common
- Pattern: Broad colour areas; some patterning
- Fragility: Very thin layer; requires protection
- Doublets/triplets: Often assembled for durability
Grading
| Grade | Colours | Description |
|---|---|---|
| AA | 3+ colours including red/violet | Exceptional |
| A+ | 3+ colours | Very good |
| A | 1-2 colours | Good |
| B | Less vivid colours | Commercial |
Ammolite Protection
Iris Quartz
Iridescence in quartz from internal fractures:
- Cause: Thin air-filled fractures
- Effect: Rainbow colours along fracture planes
- Character: Often seen in crystal slices
- Natural vs enhanced: Some is artificially fractured
Treated Iridescence
Surface treatments creating iridescence:
Coated Gems
- Mystic topaz: Titanium coating creates rainbow
- Aurora quartz: Various coating treatments
- Coated calcite: Decorative only
- Detection: Surface colour; coating may show wear
Identification Concerns
- Coatings can wear off
- Examine edges and girdle for coating
- May show interference patterns in reflected light
- Must be disclosed as treatment
Other Iridescent Gems
| Gem | Cause | Notes |
|---|---|---|
| Fire agate | Iron oxide layers | Natural; carved |
| Ammolite | Nacre layers | Natural; often doublet |
| Iris quartz | Internal fractures | Natural or enhanced |
| Rainbow obsidian | Inclusions/structure | Natural; volcanic glass |
| Labradorite | Twin lamellae | Often classified separately |
| Coated gems | Surface coating | Treatment; must disclose |