CUT:

A diamond's cut grade is about how well a diamond's facets interact with light. Cut quality refers to the proportions, symmetry and polish which ultimately results in the performance of a diamond. A diamond's cut has three primary effects on appearance: brilliance (the brightness created by the combination of all the white light reflections from the surface and the inside of a polished diamond), fire (the dispersion of light into the colors of the visible spectrum, seen as flashes of color), and scintillation (the flashes of light and dark, or sparkle, when a diamond or light source is moved).