Dispersion
Dispersion
The optical phenomenon that gives gemstones their fire
Dispersion is the separation of white light into its constituent spectral colours as it passes through a transparent or translucent medium. In gemmology, the term describes the wavelength-dependent variation in refractive index that causes a gemstone to split incident white light into a fan of spectral hues — red, orange, yellow, green, blue, and violet — visible to the observer as flashes of colour commonly called fire. Dispersion is one of the most commercially significant optical properties in the gem trade, responsible for the characteristic brilliance of diamond and the extraordinary play of colour in demantoid garnet, sphene, and a handful of other high-dispersion species.
The Physics of Dispersion
All transparent materials slow light to a speed lower than its velocity in a vacuum, a relationship expressed as the refractive index (RI). Crucially, the refractive index of any material is not a single fixed value but varies with the wavelength of light: shorter wavelengths (violet, blue) are refracted more strongly than longer wavelengths (red, orange). This wavelength dependence is called dispersion in the broader physical sense, and it is the same principle that causes a glass prism to produce a rainbow from white light.
In a faceted gemstone, this effect is amplified by the geometry of the cut. Light entering through the crown is refracted at the air-to-gem interface, internally reflected from the pavilion facets, and refracted again as it exits. Each refraction event separates the spectral components slightly further, so that the emerging ray fans out into a small spectrum. The observer perceives this as a coloured flash — fire — that shifts as the stone or the light source moves.
Quantifying Dispersion: The B–G Interval
Gemmologists quantify dispersion as the difference in refractive index between two standard wavelengths of light. The most widely used measure is the B–G interval, defined as the difference between the refractive index at the Fraunhofer B line (686.7 nm, deep red) and the G line (430.8 nm, violet). This figure, written as dnF–dnC in some European conventions (where the F and C lines of hydrogen are used instead), gives a single numerical value that allows direct comparison between gem species.
A higher dispersion value indicates that the material separates red and violet light more widely, producing more vivid and more widely spread spectral flashes. The following values illustrate the range encountered in gem materials:
- Diamond: 0.044 — the benchmark for fire among natural colourless stones
- Demantoid garnet: 0.057 — higher than diamond, producing exceptional fire
- Sphene (titanite): 0.051 — among the highest of any commonly faceted gem
- Zircon: 0.039 — notable fire, historically used as a diamond simulant
- Corundum (ruby, sapphire): 0.018 — moderate, largely masked by body colour
- Beryl (emerald, aquamarine): 0.014 — low, rarely perceptible
- Quartz: 0.013 — low
- Cubic zirconia (synthetic): 0.060 — very high, often appearing artificial in intensity
- Moissanite (synthetic silicon carbide): 0.104 — extremely high, visually distinctive
Dispersion and Body Colour
The practical visibility of fire depends not only on the dispersion value of the material but also on its body colour and saturation. In a colourless or near-colourless stone, spectral flashes are unimpeded and immediately visible against the white or silvery background of reflected light. This is why dispersion is most prized — and most commercially relevant — in colourless gem materials such as diamond, white zircon, and colourless topaz.
In strongly coloured stones, the body colour absorbs or overwhelms the spectral flashes. A deeply saturated green demantoid garnet, for example, possesses a dispersion value that exceeds diamond's, yet the fire is far less conspicuous than in a pale or lightly tinted specimen. Conversely, a fine pale demantoid can display fire rivalling or surpassing that of a comparable diamond. This relationship between saturation and fire visibility is a consistent principle across all gem species: the more saturated the body colour, the less perceptible the dispersion, regardless of the material's inherent dispersion value.
Dispersion and Cut
The cut of a gemstone profoundly influences the expression of dispersion. Facet angles, facet size, and overall proportions determine how light travels through the stone and at what angles it exits. Cuts with larger, flatter facets — such as the step cut — tend to produce broad, sweeping flashes of colour. Cuts with many small, steeply angled facets — such as the brilliant cut — break the light into numerous smaller, more scintillating flashes. The standard round brilliant cut was developed in part to optimise both brilliance and fire in diamond, and its proportions represent a carefully considered balance between the two.
For high-dispersion coloured stones such as demantoid and sphene, cutters often favour modified brilliant or mixed cuts that maximise the play of spectral colour while preserving weight and displaying the body colour to advantage. The tension between these objectives — fire, colour, and weight retention — is one of the central challenges of cutting high-dispersion coloured gemstones.
Notable High-Dispersion Gem Species
Demantoid garnet is arguably the most celebrated high-dispersion coloured gemstone in the trade. First discovered in the Ural Mountains of Russia in the 1860s, it was prized by the House of Fabergé and by Edwardian jewellers for its extraordinary fire. Its dispersion of 0.057 exceeds that of diamond, and in lighter-toned specimens the spectral flashes are vivid and unmistakable. Russian demantoid is further distinguished by characteristic curved horsetail inclusions of byssolite fibres radiating from a chromite crystal — inclusions considered a positive identification feature and, in fine specimens, a mark of provenance.
Sphene (titanite) displays a dispersion of approximately 0.051 and a strong adamantine to resinous lustre. Its fire can be spectacular in well-cut specimens, though its relatively low hardness (5 to 5.5 on the Mohs scale) limits its durability in everyday jewellery settings. Sphene occurs in a range of colours from yellow-green to brown and orange.
Diamond remains the standard against which dispersion in colourless stones is measured. Its combination of high dispersion (0.044), very high refractive index (2.417), and exceptional hardness makes it uniquely suited to cuts that exploit fire to the fullest. The term fire in the diamond trade refers specifically to the dispersive flashes of spectral colour, distinguished from brilliance (the return of white light) and scintillation (the pattern of light and dark as the stone moves).
Dispersion in Simulants and Synthetic Materials
The development of synthetic gem materials has produced substances with dispersion values far exceeding those of natural gems. Cubic zirconia, introduced commercially in the late 1970s, has a dispersion of approximately 0.060, producing fire that many observers find excessive or artificial in appearance when compared with diamond. Synthetic moissanite, introduced as a gem material in the 1990s, has a dispersion of approximately 0.104 — more than twice that of diamond — and its fire is one of the principal features used to distinguish it from diamond in visual examination. Gemmological laboratories routinely use dispersion, in conjunction with refractive index, specific gravity, and spectroscopic data, as part of the identification of simulants.
Measurement in the Laboratory
Dispersion is measured using a refractometer by taking refractive index readings at two known wavelengths and calculating the difference. In practice, monochromatic light sources or calibrated filters isolating the B and G Fraunhofer lines are used. For doubly refractive stones, both the ordinary and extraordinary rays are measured at each wavelength, and the dispersion of each ray is reported. Because the differences involved are small — typically in the third decimal place — precision instrumentation and careful technique are required for reliable results.