While the term "cut" is often mistaken for shape (e.g., pear or oval shape), the cut's quality is very different.
The unique "sparkle" of diamonds is what makes them special and highly prized. This phenomenon comes from how the diamond transmits reflected light at numerous angles. Diamonds are "sparkly" because they have multiple facets oriented at different angles. So, when you move a diamond, you see changing reflections that cause light changes and create the sparkle phenomenon.
The way the facets transmit and reflect light determines the quality of the cut. This reflective quality is determined by the precision and 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.