In those flashes of brilliance is where we see hue, tone and saturation.
Brilliancy also tells us much about how a gemstone is cut. A clean gem that is cut to good proportions will return a large percentage of light to the eye.
A poorly cut gem will normally have large areas of windowing, large areas of extinction, an imbalance or brilliance, no brilliance at all; or all of the above
Brilliance will also suggest how internal characteristics are affecting or diffusing light transmitted through the crystal.
A heavily included gem can diffuse the transmission of light, making the brilliancy appear fuzzy. Inclusions may mask large areas of the gem and not show brilliance in that area. In a gem with limited transparency, the brilliancy areas will be dim.