Dispersion Chart

Some gems display flashes of light when held and turned it in the light, which is the result of the dispersion of white light into the separate spectral colors. Dispersion occurs when white light is separated into color according to wavelength. 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.

The chart provided displays the dispersion values, which indicate the variance between the refractive indices of red and violet light, referred to as the BG Fraunhofer lines. The B line (686.7 nm) corresponds to the red portion of the spectrum, while the G line (430.8 nm) falls within the violet range. A higher difference value between these two lines indicates a greater level of dispersion.

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.