When a ray of light passes through a well-cut diamond it is refracted through a large angle and consequently the colors of the spectrum becoming widely separated strike a spectatorss eye separately so that at one moment he sees a ray of vivid blue, at another, one of flaming scarlet or one of shining green while perhaps at the next instant a beam of purest white may be reflected in his direction.

—Marcel Tolkowsky, quoted in The Book of Diamonds by Joan Dickinson, 1965.