What is dispersion in gemmology?
Definition
Dispersion is a physical phenomenon in which white light, or any other polychromatic light, separates into its different spectral colors as it passes through a prism or a refractive material.
When light enters a gem from the air, it encounters a medium with a higher refractive index. This change in medium causes a decrease in the speed at which light travels. This variation in speed causes refraction, i.e., a change in the direction of the light ray as soon as it enters the gem.
White light is not deviated as a single homogeneous entity. Each color component of the light has a different wavelength, and upon entering the gemstone, each color propagates at a different speed (more precisely, experiences a different phase shift). Shorter-wavelength rays are slowed more than longer-wavelength rays. As can be seen in the animation above, the propagation speeds vary according to color: violet rays, having the shortest wavelength, travel more slowly than red rays. This difference in speed causes a gradual separation of colors right at the first interface between air and the gemstone.
Inside the gemstone, light then propagates at a constant speed as long as it remains in the same material. However, since the different colors do not follow exactly the same path, the light rays gradually diverge from one another. The gemstone thus acts as a dispersive medium that amplifies the initial separation of colors.
To summarize
As soon as it passes from air into the gemstone, white light ceases to be a single beam. It becomes a set of distinct colored rays, each propagating at a slightly different angle, which constitutes the origin of “fire,” that is, the colorful flashes observable especially in diamond, cerussite, sphalerite, and wulfenite, which are highly dispersive minerals.
