Scintillation is the term used for the multiple and alternating reflections of light flashes (dancing sparkles) from the polished diamond, when there is relative movement between the observer and the light source or the diamond. It is the combination of surface reflection, fire (dispersion) and internal brilliance.
The brightness of scintillation depends on the proportions, angles, symmetry and polish.
Most people, when comparing diamonds in the bright lights of a jewellery store, will see mainly surface reflections. To best appreciate all of the diamonds optical effects, view it in a combination of diffused and single source white light or day light.