This reflection is known as brilliance. Brilliance which is the combination of all the white light reflections from the surface and the inside of the stone. Brilliance gives a polished diamond its brightness. Dispersion is the separation of the white light into spectral hues. Dispersion is the rainbow colors you see in a prism.
Scintillation is the flashes of light when the diamond, light, or the observer moves.
A poorly cut diamond will actually loose light and appear dull. There are two main parts to the make of a diamond. Proportions and Finish. Proportion refers to the angles and relative measurements of a polished gem and the relationship between them. A diamond s proportions are not as obvious as its size and shape, but are important to the overall appearance.