Iridescence

Iridescence in gemstones including fire agate, ammolite, iris quartz, and surface coatings with causes and identification.

By Fabian Moor Last updated
phenomena/iridescence fire-agate ammolite thin-film interference

Introduction

Iridescence is a display of spectral colours caused by thin-film light
interference at or near a gem's surface. When white light strikes a thin
transparent layer, it reflects from both the top and bottom surfaces; the two
reflected waves interfere constructively or destructively depending on layer
thickness and viewing angle, selectively reinforcing certain wavelengths.
The result is a shifting rainbow of colour that changes as the stone is tilted.
[1]

Unlike opal's play of colour, which arises from diffraction by a three-
dimensional array of ~200 nm silica spheres, iridescence is an interference
effect at planar or near-planar layer boundaries. Natural iridescence in gems
occurs when thin iron oxide layers form over a botryoidal chalcedony surface
(fire agate), or when fossil nacre layers are preserved in ammonite shell
(ammolite from Alberta's Bearpaw Formation). Surface coatings such as titanium
vapour deposition on topaz ("mystic topaz") produce the same optical effect
artificially and must be disclosed as a treatment. [2][3]

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 [2][3]

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 [3]

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

References

  1. ↑ 1. Nassau, K. (2001). The Physics and Chemistry of Color (2nd ed.). Wiley-Interscience. ISBN: 978-0-471-39106-7.
  2. ↑ 2. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224.
  3. ↑ 3. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling. ISBN: 978-1-4027-6829-3.

Related Topics

Play of Colour