The "Cut", is a combination of the diamond's proportions and its finish grade. The proportion grade represents the different dimensions of a brilliant (in %) whereas for the finish grade symmetry deviations are measured.
The proportions of a diamond determine the briliance and the fire of the stone. The proportions are even more essential for the apperance of the diamond than the clarity or the colour. A top quality diamond (loupe clean - exceptionally white +) with bad proportions will be less atractive than another diamond with worse clarity and colour but with ideal proportions. In technical terms, one speaks of "stones with (much) life" or dead stones, when brilliance is respectively present or absent.