Use of the word, "round" in referring to a diamond is in reference to the actual shape of the diamond's table and crown. The crown and table of the round diamond is cut to produce a round shape when looking down through the table. This shape is formed when the diamond is cut to bring about a tapering pavilion, leading up to a round, or circular girdle and topped off by a tapering circular crown. This shape is the most popular of diamond shapes, accounting for more than 75% of all diamonds sold.

There has been substantial research into the optimization of light, perhaps most famously with Marcel Tolkowsky in 1919, using the round shape as the focus as it is the most open to the maximization of the light display. The round diamond shape is optimized for light refraction, so that the light both enters the crown and exits the crown, thus utilizing the light for optimum fire and scintillation for the viewer. The round shape has been linked to the personality characteristics of warmth, empathy and dependability.