When light enters a diamond or any given material it bends or refracts. The degree to which it bends is called the refractive index. The refractive index of diamond is 2.41, the highest of any natural transparent gem. In diamond the maximum angle of refraction is 24.5 degrees. This is called the critical angle.
Light traveling through a diamond is reflected if it strikes a surface outside the critical angle.
Light striking a surface inside the critical angle will be refracted out of a diamond. The critical angle is represented as 24.5 degree cone in the diagram. In a well-made diamond, a high percentage of light entering through the crown starts out striking the pavilion outside the critical angle. Thus it is totally reflected to the other side of the pavilion. There it strikes outside the critical angle again, and again and is totally reflected, this time towards the crown, where it strikes within the critical angle and leaves the diamond traveling in directions in which it can be seen.