The cut determines how sparkly or brilliant your diamond will be. A properly cut diamond will refract the light that enters the diamond and return it through the top to produce the sparkle that is so desired. The typical brilliant cut diamond is cut with 58 facets, 33 on the crown and 25 on the pavilion. On a well-proportioned stone, these facets will be uniform and symmetrical. If they are not, the diamond’s ability to refract and reflect light will suffer.
The two most important numbers to be looking for when looking for great cut are the depth percentage and the table percentage.

Depth percentage is the depth of the diamond (table to culet) divided by the width of the diamond. The higher the number, the deeper the stone. The lower the number the shallower the stone. For example, a diamond with depth of 4.09mm and width of 6.75mm equals a depth percentage of 60.6%. For a round diamond, the depth percentage for the ideal cut class is 58 – 62.9%
Table percentage is the length of the table divided by the width of the diamond. The higher the number, the bigger the table looks. The lower the number, the smaller the table looks. For example, a diamond with table of 3.91mm and width of 6.75mm equals a table percentage of 57%. For a round diamond, the table percentage for the ideal cut class is 53 – 57%. Do not confuse small table with small diameter. In general, you want the bigger diameter and the smaller table percentage for the most beautiful round diamond.
Out of Round: It is interesting to note that “round” diamonds are usually not perfect circles. The length and width measurements for a round are both diameter measurements and will be different for an out-of-round shape. If the length is greater than the width by more than 0.10 mm, the diamond has not been cut well and should be avoided. For two-carat stones, the acceptable deviation is 0.12 mm.

What is Ideal Cut? There is not an industry agreement on what constitutes an ideal cut round brilliant cut diamond. Every grading laboratory has a different definition for it’s top cut grades. There is on going research by various organizations, including the GIA, to determine which specifications produce the best brilliance, dispersion, and scintillation.
At MyBellyRing.Com, we usually make our preliminary search for round brilliant diamonds that have depths of 59 – 63% and tables of 53 – 59%. Then we seek the best values in terms of big diameter, great proportions and at least Good polish and symmetry. When the information is available, we like to find diamonds with crown height 14 – 17% and pavilion depth 42 – 44%. Ultimately, we rely on our personal inspection to assess the diamond’s appearance, including sparkle and brilliance.

Fancy Shape Cut
When selecting a fancy shaped diamond, all of the 4 C’s apply as they do for a round diamond. However, there are several other considerations that are important for fancy shapes such as the overall appearance of the outline of the girdle of the stone, the “bow tie” effect and the length-to-width ratio. Currently there is no widely accepted “ideal” range of proportions for the fancy shaped diamonds. However, GIA and AGS are both doing research on this subject.

Probably the most important aspect for a fancy cut is the general appearance to your eye and that it is appealing and symmetrical. In many fancy shaped diamonds (marquise, pear, oval, emerald and heart shaped), the pavilion facets do not culminate at a point at the tip of the pavilion as they do for a round. Instead, they form an edge, called the “keel line.” This line should be centered in the diamond and this “culet” should still be as small as possible. Girdle width will vary between greater extremes on some fancy shapes compared to the round brilliant. These include the marquise, pear and heart, where the girdle tends to be thick or extremely thick at the tops of the stone and in the cleft of the heart-shaped cut. Also, the princess cut, which has square corners, may have an extremely thin girdle in these areas. Since the girdles vary with greater frequency in fancy shapes, attention needs to be paid to extremely thin and extremely thick girdles to avoid danger of chipping or excessive weight.

The bow tie effect, observed with the naked eye, is frequently found in marquise, oval, pear and some heart-shaped diamonds, and is considered to be a negative factor if prominent. It arises from the variations in the pavilion facet angles required in cutting stones which are longer than they are wide. The bow tie looks like two dark triangular shapes joined at the point in the center of the stone and looks like a man’s black bow tie. While almost every elongated stone has some “bow tie” shadowing, your goal is to find a diamond where this is minimal, absent and not a distraction. This is something that specifications do not reveal only personal inspections.

Finish Grading
There are two other grades related to how a diamond is cut.
Polish: Polish includes minor blemishes-scratches and polish lines, burn marks caused by excessive heat, rough girdles, nicks and pits, and abraded culets and facet junctions that do not influence the clarity grade. Polish marks are caused by the tiny diamond crystals in the polishing wheels used by the diamond cutters to polish the facets of the diamond after cutting. These polish marks are similar to the streaks left on a car after it is waxed.
Polish is graded into five categories: excellent, very good, good, fair and poor. Excellent means “superior,” not perfect. Thus a diamond with just a few hard-to-find polish lines or tiny blemishes would be rated excellent. If a stone has only faint polish lines or insignificant blemishes in inconspicuous places, its polish is very good. When transparent polish lines (on the crown) are visible through the pavilion, when a few facets show polish marks or when there are several small blemishes, the polish is good.

Remember, all of this is seen with the use of a 10X loupe or microscope. Obvious polish lines, polish marks on several facets, or noticeable blemishes put a stone in a fair category. And polish lines, polish marks, or blemishes which reduce transparency call for a poor rating (this is usually seen in low-clarity stones). When buying a diamond, always seek at least good polish. While very good and excellent are desirable, they are fairly rare and usually not noticeably better than those with a good rating when viewed with the naked eye.

Symmetry: Symmetry refers to the balance and alignment of the facets. When you look at diamonds, girdles are not always round and tables are not always perfect octagons. Sometimes facets are distorted, and sometimes they look as though their points have been lopped off. Sometimes the table tilts towards the girdle plane: sometimes it is off-center, and sometimes the girdle looks more like a warped record than a flat disc. The culet may not be centered, and the bezels and pavilion mains may not be directly opposite each other. These are aspects of symmetry.

Diamonds with excellent symmetry may have one or two tiny, inconspicuously symmetry discrepancies. If a stone has a few small or slight symmetry problems, its rating is very good. Symmetry issues that are noticeable under 10X call for a rating of good. If misalignments are fairly easy to see, the grading is fair. If there are any major symmetry variations, the ration is fair or poor. Just as most polished diamonds have some blemishes, most have at least a few symmetry discrepancies too. Like blemishes, most symmetry discrepancies have little effect on appearance, at least to the unaided eye.