Final answer:
The sparkle and color flashes seen in a high-quality diamond come from total internal reflection coupled with dispersion, due to the diamond's high index of refraction and strategically cut facets.
Step-by-step explanation:
A high-quality diamond sparkles with flashes of brilliant color when illuminated by white light primarily because of a process called total internal reflection. Diamonds have a high index of refraction, which means that whenever light enters the diamond, it bends at a significant angle. The critical angle for the diamond-to-air boundary is very small, only 24.4°. If incoming light doesn't strike the surface at an angle less than this, it won't escape the diamond. Instead, the light is reflected back inside, bouncing around, until it finds a path out where the angle is less than 24.4°. This constant internal reflection, combined with the often strategically cut facets of the diamond, ensures that light exits in specific places, leading to concentrated, sparkling points of light.
The brilliant colors seen are not due to the diamond absorbing or emitting colored light, but are a result of dispersion. As the white light is refracted within the diamond, its component colors spread out because different wavelengths of light bend at slightly different angles. This separation of colors adds to the diamond's sparkle with spectrums of color.