Dispersion is how differently a stone bends red and violet light. Gemologists give it as the index difference between the B line (686.7 nm) and the G line (430.8 nm).
In more detail
Dispersion is the source of fire. Diamond's published dispersion is 0.044 and moissanite's 0.104, about 2.4 times as much. Cubic zirconia's is 0.060, ruby's and sapphire's 0.018, emerald's 0.014 and amethyst's 0.013.
How we measure it
The gemstone pages use these published values. The simulator spreads each exit ray into three colors, at n − 0.3D, n and n + 0.3D. The light census uses its own fire spread Δ, about 0.86 of the published dispersion. The Cut Performance Index page lists Δ as “dispersion”: 0.038 for diamond, 0.09 for moissanite.
Related terms
- Fire: Fire is the flashes of spectral color that a cut stone sends out. In our census, fire is the share of returned light whose colors leave more than 0.6° apart.
- Fire spread (Δ): Fire spread Δ is the index difference that our light census uses to split a ray into two colors. It is about 0.86 of each stone's published dispersion.
- Refractive index: Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.
- Spectroscope: A spectroscope is a gem-testing instrument that spreads light from a stone into a spectrum. Dark lines and bands show where the stone absorbs.