Fire and Dispersion
Fire in faceted gemstones – dispersion as differential refraction by wavelength, the B–G interval, named dispersion values, relationship to facet design, and distinction from diffraction-based spectral effects.
Definition
Fire is the splitting of white light into its spectral colours (red, orange, yellow, green,
blue, violet) visible as coloured flashes in a faceted gemstone. It arises from dispersion –
the variation of refractive index with wavelength – and is enhanced by facet geometry and
viewing conditions.
Fire is not the same as play-of-colour (opal) or labradorescence: those spectral effects
arise from diffraction or thin-film interference. Fire results from differential refraction
at every facet interface.
Mechanism
The physics of dispersion and fire:
Dispersion Defined
Refractive index (RI) varies with wavelength: shorter wavelengths (violet, blue) are
refracted more strongly at any interface than longer wavelengths (red, orange). This
wavelength-dependence of RI is dispersion. When white light enters a gem, each colour
component is refracted by a slightly different angle; on exit, the colours emerge at
different positions, producing visible spectral separation – fire.
Why Dispersion is Not Diffraction
- Diffraction involves wave bending around apertures or at periodic structures
(silica sphere arrays in opal, nacre platelet stacks in pearl) – structural colour. - Dispersion involves differential refraction at an interface between two media
of differing refractive index – a bulk optical property of the material.
Both produce spectral colours but at entirely different physical length scales and by
different mechanisms. In a faceted gem, fire is produced at each facet face; in opal,
colour arises from the photonic crystal structure of ~200 nm silica spheres.
The B–G Interval
Gemmological dispersion is conventionally measured as the difference in refractive
index between Fraunhofer lines B (686.7 nm, deep red) and G (430.8 nm, violet):
Dispersion = n_G − n_B
A larger B–G value means greater potential for fire. Values are material constants,
independent of cut geometry.
Named Dispersion Values
| Species | B–G Dispersion | Notes |
|---|---|---|
| Strontium titanate (simulant) | 0.190 | Extremely high; immediately obvious excessive fire |
| Rutile (TiO₂, historical simulant) | 0.280 | Far exceeds diamond; strong birefringence also diagnostic |
| Synthetic moissanite | 0.104 | More than twice diamond; combined with birefringence is diagnostic |
| Cubic zirconia (CZ) | 0.060 | Higher than diamond; conspicuous fire. Note: 0.060 is the Read 7th-edition canonical value; the value 0.065 appears in some trade sources without primary citation and is not used here |
| GGG (gadolinium gallium garnet) | 0.045 | Higher than diamond; obsolete simulant |
| Diamond | 0.044 | High for natural gems; the benchmark reference value |
| Demantoid (andradite garnet) | 0.057 | Higher than diamond; contributes to demantoid's famous fire |
| Sphene (titanite) | 0.051 | Very high; softness limits durability for jewellery |
| YAG (yttrium aluminium garnet) | 0.028 | Moderate; obsolete simulant |
| Almandine garnet | 0.027 | Moderate; low fire contributes to darker appearance |
| Zircon (high type) | 0.039 | Moderate fire; good dispersion for affordable stone; also birefringent |
| Sapphire (corundum) | 0.018 | Low; fire not a notable feature of sapphire |
| Topaz | 0.014 | Low; relatively little fire |
| Quartz | 0.013 | Low |
| Fluorite | 0.007 | Very low; glass-like appearance |
Facet Design and Fire
Fire is not purely a material property – cut geometry strongly determines how much
dispersion is visible:
Cut Geometry
A brilliant cut allows white light to enter through the table, undergo total internal
reflection at the pavilion facets, and exit through the crown facets. Each reflection
and refraction event disperses the light further. The crown height, facet angles, and
number of facets all influence how the dispersed colours emerge to the viewer.
Very shallow or very deep pavilion angles reduce total internal reflection and therefore
reduce both brilliance and fire.
Trade-off with Brilliance
High dispersion often correlates with high RI (both increase as electron density
increases). However, brilliance (white light return) and fire compete in cut design:
- A broad table suppresses fire (less crown facet area to produce coloured flashes)
- More and smaller crown facets generate more fire at the cost of some brilliance
- The classic round brilliant optimises both for diamond
Distinguishing Fire from Other Spectral Effects
| Effect | Mechanism | Visual Character | Host Material |
|---|---|---|---|
| Fire (dispersion) | Differential refraction at gem facets | Coloured flashes that change with viewing angle; seen in faceted gems | Faceted stones |
| Play-of-colour (opal) | Diffraction from silica sphere array (~200 nm) | Spectral colour patches shifting with viewing angle; cabochon or rough | Precious opal |
| Labradorescence | Thin-film interference in Bøggild intergrowth | Directional broad colour zones in feldspar; not rapid flashes | Labradorite feldspar |
| Orient (pearl) | Thin-film diffraction/interference in nacre platelets | Soft iridescent surface bloom; not rapid flashes | Nacre-covered pearls |
| Iridescence (general) | Thin-film interference or diffraction | Broad spectral sheen associated with surface or near-surface structure | Fire agate, ammolite, surface films |
Diagnostic Relevance
Using dispersion in gem identification:
High Fire as Diagnostic
Fire conspicuously greater than diamond narrows identification to a short list.
Combine dispersion observation with RI, SG, and optic character:
- Excessive fire + birefringence (doubling of facet edges) = moissanite
- Excessive fire + high SG, no birefringence = CZ or GGG (historical)
- Extreme fire + very high birefringence = rutile or synthetic rutile (historical)
Sources
Read (2008)
Gemmology (3rd ed.). Butterworth-Heinemann/Routledge. DOI: 10.4324/9780080507224. [APPROXIMATE – chapter confirmed; page references not independently verified] – Primary source for dispersion mechanism and B–G values. No single DOI-verified comprehensive dispersion table paper was located in the source research session; values are textbook-consensus and should be verified against a primary spectrophotometric source before formal examination use. [Confidence C for the full dispersion table]
Nassau (2001)
The Physics and Chemistry of Color (2nd ed.). Wiley-Interscience. No DOI retrieved. [UNVERIFIED] – Conceptual support for dispersion mechanism.