The Round, or Brilliant Cut Diamond is, by far, the most popular shape for diamond engagement rings. It is cone-shaped to maximize light return through the top of the diamond. It is cut to have 58 facet - 33 on the crown and 25 on the pavilion. The relationship between the angle of the crown (above the girdle) and the pavilion (below the girdle) is complementary. A steep crown angle is complemented by a shallower pavilion angle & vice versa.
