Cut
Many people confuse "cut" with shape (round, oval, princess, pear), but instead, cut relates to the arrangement of angles, proportions, and tiny facets that determine how light enters, reflects, and returns to your eyes.
A perfectly cut diamond will have three types of sparkle: Brilliance (white light), Fire (rainbow flashes), and Scintillation (the sparkle you see when it moves). When a cut is too shallow or too deep, light leaks out instead of bouncing back, and can make the stone look dull.
Diamond certificates usually grade a cut as Excellent (or ideal), Very Good, Good, Fair, or Poor. If you're looking for a diamond that truly sparkles, prioritise Excellent or Very Good, as even a smaller diamond with a great cut can outshine a larger one with a poor cut.