Diamond proportion
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. H

ere 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.
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
Diamond.com'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 roun

d 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

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