By far the most preferred diamond shape, the round-brilliant cut is also the most optically brilliant because of its 360-degree symmetrical shape. A round brilliant is a great choice if you want the most sparkle and the most enduring classic shape. The round shape has been cut for centuries, but in 1919, Marcel Tolkowsky defined specific angles and proportions to yield the most brilliant diamond, which form the basis for the modern 'Ideal' cut round-brilliant diamond. Round-brilliant diamonds are the only shape to have this ideal proportion defined. The round silhouette works with almost every mounting, from classic solitaires to the most avant-garde designs. Rounds can be set into four or six prongs, based on the design of the setting, or into bezel mountings (a metal band that runs around the edge of the diamond to hold it securely in the setting). In general, if the round brilliant has an Ideal cut or Very Good cut, you want the setting to have the least amount of metal around the stone so that it is held securely but does not cover up too much of the diamond and block light from entering the stone.

The princess cut is a modern classic of clean, square lines and beautiful sparkle. This shape is the perfect choice if you prefer a square or rectangular outline but want the brilliance of a round. Developed in the 1970s, the princess cut is now second only to the round brilliant in popularity. The cutting of this diamond combines the step-cutting of the emerald cut with the triangular facets of the brilliant cut and is cut with right-angle corners. Although most prefer a square outline, some stones are cut with a slightly more rectangular outline. The princess cut works beautifully as a solitaire but also looks great paired with side stones, especially trillions or smaller princess-cut diamonds. It is important to protect the more vulnerable corners with a V-shaped prong at each point.

The marquise cut is a regal, elongated shape with tapering points at both ends. Its shape tends to flatter the finger, making it appear longer. When choosing a marquise cut, the length-to-width ratio should be considered. Usually a ratio of 2:1.0 is preferred, meaning that the length of the stone should be about 2 times the width of the diamond. However, like all fancy shapes, personal preference prevails, and some may prefer a shorter, wider outline or longer, thinner shape. Just look for good symmetry to ensure overall beauty no matter what outline you prefer. This shape works in a simple solitaire setting or looks beautiful with side stones, especially baguette or trillion shapes. A marquise-cut diamond should be mounted with six prongs: four positioned on the sides to hold the body of the stone securely and two V-shaped prongs to protect the points at either end, the most vulnerable part of the diamond.

The emerald-cut diamond is among the most classic of diamond shapes. Its clean lines come from step-cutting, or parallel line facets. It is always cut with blocked corners and is usually cut to a rectangular outline, although a few are cut to be more square. Because of its simpler faceting structure, larger inclusions are sometimes more visible to the unaided eye, so diamonds cut in this shape usually need to be higher clarity (I1 or I2 clarities should probably be avoided). Length-to-width ratios should be considered when choosing an emerald cut: Usually a 1.50:1.00 ratio is preferred, meaning that the length of the stone should be about 1½ times the width of the diamond. However, like all fancy shapes, personal preference prevails, and some may prefer a squarer outline, or longer, thinner rectangle. An emerald cut is loved by purists and looks especially wonderful set in platinum, in a simple setting or a baguette side-stone setting.

The radiant cut is a beautiful combination of the classic emerald cut and the sparkle of the round brilliant. The radiant cut is similar to the princess cut but is usually (though not always) a more rectangular outline and has blocked corners like those of an emerald cut. The cutting is a combination of the step-cutting of the emerald-cut diamond with some triangular faceting of the brilliant cut. The radiant cut is dramatic as a solitaire but also looks great paired with side stones such as baguettes, trillions, or princess shapes. A radiant-cut stone should be set with special prongs to hold the blocked corners securely.

The pear shape is a beautiful, feminine diamond shape with a rounded end on one side and a tapering point at the other. It is lovely as the center stone in a ring or outstanding as a pendant or pair of drop earrings. As with many fancy shapes, length-to-width ratio should be considered. Usually a ratio of 1.5:1.0 is preferred, meaning that the length of the stone should be about 1½ times the width of the diamond. Some may prefer a shorter, wider outline or a longer, thinner shape. Good symmetry is a must for pear-cut diamonds. This will ensure that light is reflected evenly, especially in the point. The asymmetrical shape should be considered when setting a pear cut, which looks beautiful as a solitaire, or with side stones, especially smaller pear-cut stones or baguettes. A pear-shaped diamond should be mounted in a special setting with five prongs: two to hold the rounded end, two to hold the curved sides of the stone securely, and one V-shaped prong to protect the point at the other end, the most vulnerable part of the diamond.

The oval cut is most similar a round-brilliant cut and combines the round's sparkle with a flattering, elongated outline. It makes a good choice for someone who wants a unique shape but loves the fire and brilliance of a round diamond. The length-to-width ratio of ovals can vary based on personal preference. Generally a ratio of 1.5:1.0 is preferred, meaning that the length of the stone should be about 1½ times the width of the diamond. However, like all fancy shapes, personal choice should guide you; some may prefer a shorter, wider outline or a longer, thinner shape. The relatively symmetrical shape lends itself well to a variety of mounting styles. Most oval cuts look great in any mounting meant for a round brilliant as long as the setting that holds the diamond has six prongs properly spaced for security.

The heart-shaped diamond is the most romantic of diamond shapes. It is similar to the pear shape but has a cleft in the rounded end that forms the lobes of the heart. The complexity of the shape requires skilled cutting to ensure proper brilliance. Symmetry is a big consideration for this shape, as the outline needs to have a pleasing, obvious heart outline apparent in the setting. The lobes should be rounded (not pointed) and clearly defined. Heart-shaped diamonds should be mounted in special settings with five prongs: two at the lobes of the heart, two on the sides of the heart, and a V-shaped prong to protect the point of the heart, the most vulnerable place on the diamond.

This elegant shape is a variation on a classic emerald cut, developed in 1902 by the venerable Asscher brothers in Holland. The Asscher cut is not a traditional choice but has gained deserved exposure recently because of celebrities wearing the cut. The uniqueness of this shape is defined by the pavilion, or bottom part of the diamond, that has a 'scissor cut' with all facets step-cut down toward the culet, or point on the bottom. The blocked corners add to its geometric appeal, making the diamond appear almost octagonal. It is usually cut to a square outline as opposed to rectangular. This diamond shape is beautiful in a simple solitaire or in a setting that has simple geometric lines or side stones such as baguettes. Ideally, the setting for an Asscher cut should not hide the unique blocked corners.

The cushion cut is an unusual diamond shape and an interesting alternative to an oval- or princess-cut diamond. Because these are relatively rare, this shape is for someone who wants something few people possess. The modern cushion shape is based on an antique cushion cut, which is a combination of round and square outline with a softened square or 'pillow' shape. A cushion-cut may be squarer with length and width in equal proportion, or may have a slightly elongated outline, depending on the individual stone and the wearer's preference. As a solitaire, it makes a statement and also looks wonderful paired with side stones such as baguettes. A cushion-cut diamond setting should have at least four secure prongs.

The trillion cut is a dramatic cut that makes a bold statement. It was developed in the 1970s as a variation of the radiant cut, combining step-cutting and brilliant faceting. While often used as side stones, this cut is rarely used for the center diamond, so it is somewhat scarce in larger sizes. The triangular shape needs to be considered with the style of mounting and would probably go best in a simple solitaire or geometric setting that works with the unique shape. The trillion cut will require a special setting that has V-shaped-prongs to protect the corners of the diamond.

The quality of a diamond is determined by the combination of Color, Clarity, Carat Weight and Cut (the “Four C’s”). These factors have varying degrees of impact on price.

Of all the Four C’s, Cut is the factor that has the most dramatic impact on the price of the diamond. Cut unlocks the beauty of the finished diamond from the rough diamond crystal, and determines how much it will sparkle. “Sparkle” is determined independent of how large it is, or the color or clarity grade of the diamond.

The color grade has the next largest impact on price, as the body color of the diamond can potentially be visible to the unaided eye.

Clarity typically has the smallest impact on price, since to see the differences for clarity grades SI2 and above you need 10 power magnification.

Carat weight does affect price significantly, but the other factors play a part in this—a larger diamond does not necessarily cost more than a smaller diamond. Only if all other factors are comparable will a larger diamond will be more expensive than a smaller carat weight.

However, even with all this in mind, remember that a diamond is a package deal of all the factors combined—the sum of its parts. There may be some trade-offs necessary to balance price with the quality factors that are most important to you, but this should still result in a diamond with beautiful sparkle.