What Causes Gemstone Dispersion?

Normal white light is composed of a rainbow of colors. Each of those colors is a wavelength of light. When white light passes through a gemstone, each of its colors travels through it at a different speed. This is literally “dispersion” — the wavelengths previously joined in white light “disperse” and exit the gem separately, creating the rainbow color display. For example, blue is a short wavelength of light and travels slowly through a gem. Red is a long wavelength of light and travels more quickly through a gem than blue.

How Do You Measure Gemstone Dispersion?

Although you can appreciate gemstone dispersion strictly on an aesthetic level, gemologists can measure it with a refractometer and use that information to help identify a gem. The higher the measurement, the more dispersion the gem can display.

Every gem species refracts the distinct wavelengths of light (colors) at different speeds. Gems have a refractive index (RI) for each wavelength of light. Typically, gemologists use a yellow light source (589 nm) to measure RIs. However, to measure a gem’s dispersion, gemologists must calculate its RI using red light (686.7 nm), then violet light (430.8). The absolute difference between a…