Cut is the only diamond characteristic directly influenced by man - the other three are dictated by nature. A good round brilliant cut releases the life, fire and sparkle of a diamond through the arrangement and proportions of its 57 or 58 facets (tiny planes that create angles to reflect light).
When a diamond is cut to good proportions, light will reflect from one facet to another and disperse through the top of the stone, resulting in display of brilliance and fire.
The shape of a diamond is a matter of personal taste, with the round brilliant cut the most popular of the shapes. Other shapes include the emerald cut, pear shape, marquise, princess cut, oval, heart shape and cushion cut.