The cut of a diamond is one of the fundamental attributes (one of the 4 C's) to knowing how to judge a diamond. Although it is often confused with the Shape of the diamond (Round cut, Princess cut, Emerald cut), it actually refers to how well the stone is polished from rough. A Premium cut gives a diamond a higher level of sparkle intensity. The angles and finish of the diamond determine its ability to reflect light. A better polish means a better reflective quality and hence a better sparkle.
Proportions determine a diamond's brilliance (the amount of light reflected back to your eye), fire (the flashes of colour due to prismatic separation into the colours of the rainbow) and scintillation (sparkling movement of light as you move the diamond). If ideal proportions are used in the creation of diamonds, their overall excellence can be optimised.
In 1919 Marcel Tolkowsky was responsible for discovering the basis of what is now regarded as the “Ideal Cut” diamond. The Mathematician formed a Masters thesis on the proportions for round brilliant cut diamonds.
Supposedly, he conducted his research by asking Londoners to select the most appealing diamond from a small group. He combined these observations with those of his family’s Belgian diamond cutter business and then applied maths and physics to confirm why certain proportions produced the best looking diamonds.