Cut

The cut is another tangible grading scale, which refers to the style of the Diamonds cut (the shape). Diamonds can be cut into a variety of calibrated and non calibrated shapes and sizes. Each cut is designed to bring the best out of a stone, making it refract the light to its greatest potential, thus making it sparkle.
A Diamond is cut into a variety of shapes, with brilliant cut (round) being the most popular, other cuts available are Pear, Trilliant (triangular), Princess (square), marquise and Emerald plus other trademarked cuts developed by companies for their own use.
If calibrated cuts are cut correctly (brilliant, princess etc.), a Diamond can reflect and refract large amounts of light back out through the table and the top of the stone, which creates that sparkle and fire that diamonds are so famous for.
The type of cut will be gauged accurately on a diamond certificate, the quality of the cut will also be graded, normally on the scale of poor through to excellent. How well the diamond is cut is arguably the most important of the four C's, as it can make an average coloured Diamond look good or adversely, a stone with good colour and clarity look bad.