When you hold a gemstone and turn it in the light, some gems will display flashes of color, which is the result of the dispersion of white light into the colors of the spectrum. Dispersion occurs when different wavelengths of light are refracted by a different amount by the refracting medium. Diamond is famed for this quality, but a number of other gemstones are notable for their fire, including demantoid garnet, sphene and zircon.
Dispersion can be measured with a refractometer. Dispersion is usually expressed numerically as the difference between the red and violet refractive indices.
In the following chart, the values for dispersion are computed using the refractive indices at the wavelengths represented by the so-called B and G Fraunhofer spectral lines. The B line (686.7 nm) is in the red part of the spectrum, while the G line (430.8 nm) is in the violet range. The higher the value, the greater the dispersion for the gem variety.
However, one shouldn't rely on the chart alone to determine whether a gemstone will display significant fire. Gems with a darker body color will exhibit less fire than lighter colored gems. In general we can say that large light colored stones with very good clarity will display the greatest dispersion.