Understanding Cut
The only one of the 4Cs that is within our control is a diamond's cut. Cut is the result of a craftsman’s skill in transforming a rough diamond into a breathtaking gem. A diamond has facets that allow light to enter it, become refracted, and exit in a rainbow of colors. So it follows that a better cut diamond does a better job of dazzling us with its beauty. Here’s the ‘secret’ to how a diamond sparkles:

As you can see, the diamond with the correct proportions does a more effective job of refracting light out the top of the stone to your eye. The less-than-ideal cut diamonds allow some light to become lost out the bottom.

There are many measurements that go into creating a diamond that truly maximize the refraction of light. Facets must be cut at exactly the right angles relative to one another; the top and bottom halves of the stone must have the proper depth relative to each other; the table, or flat surface on the top, must be the correct size, relative to the overall size of the stone. And, of course, the facets on the top (crown) and bottom (pavilion) must align correctly with each other. Here is a diagram showing a diamond’s basic proportions:

These measurements and percentages are different for each diamond, and are all taken into account in the evaluation of a diamond’s cut. In fact, each diamond shape (heart, round, oval, emerald, etc.) has its own set of guidelines for what makes a Premium or Good cut.

Another factor you’ll need to consider when choosing a diamond is its shape. Although ‘shape’ and ‘cut’ are sometimes used interchangeably, they are not the same. There are eight popular shapes into which diamonds are crafted:

Round Brilliant - the most classic cut
Marquise - an elongated brilliant-cut stone with a point on each end
Princess - typically a four-sided square to slightly rectangular brilliant cut
Radiant - typically a slightly rectangular to square diamond
Emerald - a traditional octagonal cut usually rectangular
Asscher - often referred to as a “square emerald cut,” this stone is step-cut and square, with cropped corners
Pear - combines the brilliance and form of a round stone with the elongated elegance of a marquise
Oval - reminiscent of the round brilliant cut, both in sparkle and shape
Heart - more fanciful cut, shaped just as it sounds

The Ideal Cut Diamond
An 'ideal cut' is a specific set of guidelines that delineate the proportions that give a diamond the highest amount of fire and brilliance.

Although the proportions of an ideal cut vary depending on the source you talk to (from jeweler to jeweler, country to country), there are certain ranges that are generally accepted as capable of evoking the most desirable fire and brilliance from a stone. These ranges must cause the light entering the diamond to be reflected and dispersed through the table (top), not through the sides or bottom. But most of all, an ideal cut diamond must be cut to bring out the stone's brilliance and fire, not retain the most weight from the rough cut stone.

Fine Silver & jewelry's Diamond Proportion Guidelines
Diamond.com maintains extremely stringent guidelines for diamond quality. All diamonds we offer must fall within specific ranges to be judged acceptable by our staff gemologists.

Below is a table listing each of our three cut grades for a round brilliant diamond (Ideal, Premium and Good). The table percentage given is the measurement of the diamond's table width, relative to the width of the entire stone (at its girdle, or widest point). This relationship is critical for maximum fire and brilliance: too small or too large a table can hamper the diamond's ability to disperse light properly, not to mention possibly causing the stone to appear smaller for its weight.

The depth percentage listed is the length of the stone from table (top) to culet (bottom), expressed as a percentage of the diamond's width, measured at its widest point. This depth-to-width relationship is largely responsible for the diamond's being properly proportioned, resulting in the proper refraction of light and maximum fire and brilliance.

Round Brilliant Diamond Cut Grades

GRADE TABLE PERCENTAGE DEPTH PERCENTAGE
Ideal 53% - 57% 59% - 62%
Premium 58% - 63% 58% or 63%
Good 64% - 65% 57.5 or 64%

It's important to note that some independent grading laboratories do not grade a diamond's cut, while others do. The Gemological Institute of America (GIA), for example, does not give an evaluation of the quality of a diamond's cut, only the shape and measurements of the stone. It does, however, give the proportions of the diamond (depth and table percentages, girdle thickness, culet size (if any) as well as a general rating of the stone's finish, including its polish and symmetry.

Symmetry
Symmetry describes several factors: how the facet edges align with each other; whether the corresponding facets from opposite sides of the diamond align with each other or not; and whether the facets from the crown, or top of the diamond are properly aligned with the corresponding ones on the bottom (pavilion). When choosing a diamond, look for Excellent, Very Good or Good ratings on the grading report, if symmetry is graded.

Shape
Of course, the most popular and commonly seen shape for diamonds is the round or brilliant cut, which has 57 or 58 facets (depending on whether the culet, or point at the bottom of the stone, is faceted or not). It's the 'classic' shape that most people think of when they think of a diamond. But the round cut is by far not your only choice. All shapes are capable of fantastic fire and brilliance. The only difference is the result of the diamond cutter's decision to cut the rough diamond into that particular shape.

Choosing the best shape for you
The most important factor in determining shape, of course, is what appeals to you, and what looks best on your hand. Longer and shorter stones can visually affect the appearance of your hands, making them look longer or shorter in return. Also, your taste may guide you toward more traditional shapes, like the classic round brilliant, or toward less conventional shapes like pear, marquise or heart. While the shape of the diamond you choose is ultimately a matter of personal preference, there are differences in the various shapes that affect their brilliance, apparent size and value:

Round brilliant-cut diamonds show the most brilliance and sparkle of all the shapes. When it comes to hiding imperfections, the round brilliant cut is the king. Its design allows it to hide flaws and yellow tints better than diamonds of other shapes. In emerald cuts and baguettes, which have long, flat facets, flaws become the most obvious.
Emerald cuts, while sleek and attractive, are not quite as brilliant. If you like the square or rectangular shapes of an emerald cut, you may want to consider a radiant, princess or quadrillion, which have more facets and therefore more brilliance.
If you want a diamond that looks as big as possible, even if it doesn't weigh much, consider a fancy shape like a marquise or pear, which appear bigger and longer than round diamonds of the same carat weight.

Fluorescence
When exposed to ultraviolet light, small percentages of diamonds fluoresce, or emit light, which may be yellow or blue. Fluorescence does not necessarily affect a diamond's value, however it is listed on a diamond grading report.

The bottom line
When it comes to judging cut, the most important thing to remember is that quality and craftsmanship are more important than size, since these characteristics can mean the difference between a positively radiant, lively diamond, and a lifeless, lackluster stone (that may, in fact, be larger). Remember, diamonds have more value if they have been cut to maximize their brilliance, not their size. All in all, cut must be balanced along with the rest of the 4Cs in order to find the highest quality diamond your budget will allow.