While the term “cut” if often mistaken for shape (e.g. pear or oval shape), the quality of the cut is actually very different.
The unique “sparkle” of diamonds is what makes them special and highly prized. This phenomenon comes from the way that the diamond transmits reflected light at multiple angles. Diamonds are “sparkly” because they have multiple facets which are oriented at different angles. So when you move a diamond, you see changing reflections. This causes the changes in light and creates the sparkle phenomenon.
The way that the facets transmit and reflect light determines the quality of the cut. This reflective quality is determined by the precision & symmetry of the cuts as well as the smoothness of the polish. To determine the cut grade of a standard round brilliant diamond, GIA calculates the proportions of the facets that influence the face-up appearance.