The shape of a diamond has a lot of influence over the feel of an engagement ring, and for the best aesthetic, the lines of the diamond should reflect the design, or vice versa.
Round diamonds are the traditional choice for engagement rings, and remain the most popular. A round diamond is cut to mathematically return the most light to your eyes, and so it appears the most sparkly.
Next in popularity to the round are the cushion and the princess cut diamonds. Historically, the princess cut was the second most popular diamond, however, the cushion cut diamond has received much attention in the last decade and is in a tight running for second place in popularity.
The princess cut diamond is a square, and that hard edged geometric shape lends itself to more modern designs. The best setting for this shape is a v-tip prong, which protects the corners and securely holds the stone.
The cushion cut diamond can have a square ratio or it can be rectangular, and each cushion cut diamond is unique, in that there can be variation in the curvature of the sides and how rounded the corners are. It is a soft and unique shape that lends itself well to a gorgeous halo design.
The oval diamond is more popular in the vintage engagement ring styles, although it is a popular choice for three stone rings as well, as you can get great height, and still squeeze in three stones.