When discussing the visible spectrum of light, it was noted that each of the primary hues had a different wave-length. Although light travels in air at the same velocity regardless of wavelength, when it enters a gemstone there is a difference in the velocity of the various wavelengths. The shorter the wavelength of light, the more it is slowed down by the gem. It was also pointed out in earlier discussions that the amount of refraction, or bending of light, depends on the angle of incidence and the change in velocity that takes place when light passes from one medium to another of different optical density. Since each wave-length has a different velocity in a gemstone, if light impinges on a prism at an oblique angle, each wavelength will be bent a different amount and the light will be separated into its component hues, referred to as a SPECTRUM. The longest wavelength, red, is refracted the least, and that violet, the shortest wavelength, is refracted the most. For simplification, the R.I. for a substance is usually given for a very narrow range of yellow light in the middle of the visible spectrum.
- The breaking up of white light into its component colors is known as DISPERSION. Dispersion is measured by the difference in the refractive indices of certain of the red and blue wavelengths of light as they pass through a given material.
In gemology, so that comparable figures can be given for dispersion, specified wavelengths in the red and blue ends of the sun's spectrum are used; these are referred to as the B and G Fraunhofer lines. For example, the R.I.'s for these two wavelengths in spinel are 1.710 and 1.730 respectively, therefore, the dispersion is 0.020.
In general, dispersion in gems increases with increasing refractive index. However, there are exceptions to this, as shown on the refractive index and dispersion tables accompanying this assignment.
Scintillation
Scintillation in gemstones may be broadly defined as a flashing of light from the polished facets of a stone that are seen by the observer as the gem, the illuminant or the observer moves. The more movement, the more scintillation. The comparative degree of scintillation for a given gemstone is determined primarily by the number of facets on the stone that, will reflect light to the eye in a number of individual flashes as the stone is moved about. However, the sharpness and clarity of the "sparkle" is affected by the quality of the polish possible on the facets, since the more highly polished the facet, the less diffused and stronger will be the flashes.
Although scintillation is more important in diamond, due to its superior reflective surfaces, it is also a factor to be considered in colored stones. A maximum degree of scintillation might not, however, produce a maximum degree of beauty. On small stones, the placing of a very large number of facets can reduce the surface area of each one as to make them difficult to observe individually, thus producing an overall "fuzzy" appearance. Clearly, the most desirable condition is one somewhere between the broad flashes of white light from a stone with large facets and one with facets so tiny that they appear fuzzy.
