Cut:
The quality of a diamond's cut is widely considered the most important of the 4 C's in determining the beauty of a diamond, indeed, it is commonly acknowledged that a well-cut diamond can appear to be of greater carat weight, and have clarity and color that appear to be a better grade than they actually are.

The cut enables the diamond to reflect light off of it's various angles, giving it, it's fire and brilliance. Though nature may determine a diamond's clarity, color, and carat weight, the hand of a craftsman is necessary to release the diamond's fire, sparkle, and beauty.

Incorrectly cut diamonds, lose light through inadequately formed angles, thus losing brilliance, beauty, and even value.

To review the different styles and shapes of cut diamonds, refer to the table below.

CUT DESCRIPTION
Round Brilliant: The most common style of cutting both diamonds and colored stones. The standard round brilliant consists of 57 facets; 1 table, 8 bezel facets, 8 star facets, 16 upper-girdle facets on the crown; 8 pavilion facets, 16 lower girdle facets; and usually a culet on the pavilion.
Pear: A variation of the Brilliant cut, with 58 facets to only 56 facets. Shoulders should have a gently but distinctly rounded arch. Common length-to-width ratio: 1 to 1.50-1.75.
Oval:  A brilliant style of cutting very similar to a Round except it is elliptical. It was invented by Lazare Kaplan in the early 1960s. Oval brilliant usually has 56 or 57 facets. Beware of uneven or high shoulders (they should have a gently but distinctly rounded arch). Typical length-to-width ratio: 1 to 1.30-1.65.
Marquise: This shape has a boat shaped girdle with 57 facets. The shape and placement of the facets is of the brilliant type. Look for uneven "wings" or undefined points. Typical length-to-width ratio: 1 to 1.75-2.25.
Emerald: It is usually rectangular but sometimes may be square, in which case it is known as a square emerald cut. It has rows (steps) of elongated facets on the crown and pavilion, parallel to the girdle, and with corner facets. The number of rows of elongated facets may vary, although the usual number is three on the crown and three on the pavilion. Inclusions are slightly more visible in "step-cut" shapes relative to "brilliant" styles. The beveled corners protect the stone and make it easier to set. Typical length-to-width ratio: 1 to 1.30-1.70.
Princess (also the patented Quadrillion): Is a relatively new shape with at least 45 facets (no culet) normally close to a square shape (+ or - 10%), but may come in elongated versions. Watch out for girdles which are extremely thin and thus prone to chipping. Typical length-to-width ratio: 1 to 1.00-1.15.
Radiant: Patented cut with 70 facets (often confused with "cut corners Princess/Quadrillions").  Typical length-to-width ratio: 1 to 1.20-1.50.
Heart: Look for uneven or flat "wings" or too shallow cleft. Typical length-to-width ratio: 1 to 0.90-1.10.
Trillion: Popular choice for side-diamonds to enhance center diamond. Typical length-to-width ratio: 1 to 0.90-1.10.

To learn more about the 4 C's, click on the respective link below:

Cut Color Clarity Carat Weight