Brilliance refers to the brightness of a diamond, created by the combination of all the white light reflections from the surface and the inside of a polished diamond to the eye. Contrast refers to the light and dark areas seen in a diamond. The dark areas are not inclusions, they are shadows created by the cut of the diamond or objects between the light source and the diamond (such as the observer's head).

The greater the amount of reflected light, combined with strong contrasts, the more brilliant the stone. A round diamond will typically exhibit more brilliance than a fancy shape due to the superior mechanics of the round cut for reflecting light.

Cut is the most important factor in achieving brilliance in a diamond.