A good cut gives a diamond its brilliance.
Cut refers to the proportions of the diamond as opposed to its shape (round brilliant, marquise, pear, princess, etc.) Every diamond regardless of its shape gets it brilliancy and scintillation by cutting and polishing the diamond facets to allow the maximum amount of light to enter through its top and to be reflected and dispersed back through its top again.
Rough diamonds, those that are mined from the earth, do not display all their beauty. They need to be cut and polished to exhibit the fire and brilliance that diamonds are known for.
The most popular shape for diamonds is the round brilliant cut with 58 facets. Stones that are not cut into this shape are known as fancy cuts. The decision of the cut of the diamond is often decided by the original shape of the rough stone, location of the inclusions, flaws to be eliminated and the preservation of the weight.
The drawing shows the play of light inside a diamond and explains why an ideal cut diamond has the maximum luster and fire.