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 colour 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.
In the following chart, the values for dispersion represent the difference between the red and violet refractive indices, which are known as the BG Fraunhofer 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 of difference between the two, the greater the 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.
- First Published: November-23-2006
- Last Updated: May-29-2014
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