The prettiest diamond is the one that catches your eye and makes you want to own it. It is the one that looks best on you, whether it is big or small, expensive or not. Every person's idea of the prettiest diamond is different, so there is no definitive answer. However, some diamonds are generally considered to be more beautiful than others, such as those with perfect symmetry or those with a rare color. Ultimately, it is up to each individual to decide what they consider to be the prettiest diamond.