For any diamond, you’ll want to find one with a fantastic cut quality. Although a poorly cut diamond will still have some sparkle, you’d be better off opting for a smaller, well-cut diamond. The smaller one will perform better and can appear larger!
While judging cut quality for fancy shapes is somewhat subjective, evaluating round brilliant diamonds is almost a science. If you’re looking for a round diamond with the best sparkle, limit yourself to “Excellent” or “Very Good” cut grades.
Make sure that the stone has good sparkle throughout. Look at the brilliance (white flashes) and dispersion or fire (colored flashes) in the diamond. A well-cut diamond should have a good mix of both, but you may prefer more fire or brilliance.