Cut refers to the precise proportions and dimensions (the form of the diamond) and determines how a diamond handles light. The proper angles of cut allow light to internally reflect within the stone and allow the stone to have more brilliance and fire to sparkle better. A poor cut will make the stone dull to the eye.
At the turn of the last century, it was mathematically determined, what the optimal proportions for a diamond should be to assure maximum brilliance and dispersion. Those proportions are as follows:
The quality of a diamond's cut makes all the difference in how the stone appears. A poorly cut "D", Flawless stone, can look dead and lifeless, whereas an ideally proportioned diamond of much lower color and clarity will appear radiant and dazzling. So, why aren't all diamonds ideally proportioned? The answer is quite simple. Diamonds are sold by weight. Stones cut to ideal proportions waste more of the diamond crystal, therefore weigh less than stones that are cut to maximize weight.