A diamond cut refers to the symmetry and proportions created when a cutter transforms a diamond into its final polished state.
The cut of a diamond affects its ability to reflect light (in other words, its brilliance), and therefore a poor cut will make the diamond less luminous. In its rough state, a diamond is fairly unremarkable in appearance, so the cut is vitally important in unleashing the stone's beauty.
A number of different cuts, including round brilliant and princess, have been developed in order to give the best sparkle. Cut is one of the four Cs of diamond quality, and has probably the greatest impact on a diamond's appearance.
A cut is a symmetrical arrangement of facets, which in combination modify the shape and appearance of a diamond crystal.
Polish and symmetry are the two important aspects of a cut. The polish describes the smoothness of the diamond's facets, and the symmetry refers to how those facets are aligned.
Good cuts reflect light internally, from one facet to another, dispersing it evenly through the crown. A diamond that is cut too shallow and wide will lose light through the bottom, and if cut too deep and narrow, it will lose light through its sides.
The round brilliant, developed around the year 1900, is the most popular diamond shape and cut.
Around 75 per cent of diamonds sold are believed to be of this type. This is probably because it reflects light best and allows diamonds to be the most brilliant for their size. Most rough diamonds are also shaped in such a way that two round brilliant diamonds can be cut with the least amount of crystal lost, so this could help to explain the prevalence of the round brilliant cut.
The princess cut diamond, first created in 1980, is another popular cut, particularly for engagement rings. It is seen as a flexible choice which tends to have a slightly lower price-per-carat than round cut diamonds. The princess is traditionally a square cut, but is sometimes found to be rectangular in lower-cost examples.
Brilliant cuts are often modified into different shapes. Oval cuts boast an elongated shape which can create an illusion of a larger-sized diamond, and a similar example is the marquise cut, which is shaped like a football. Other shapes which tend to rely on fashion trends for their popularity include pear-shaped, cushion, asscher, radiant and heart-shaped.
An alternative to the brilliant cut is the step cut, which has sloping, four-sided facets that are cut below the table and run parallel to the diamond's girdle. Step cut diamonds have fewer facets than brilliant cut diamonds. Emerald cuts are step cuts with their corners clipped off.
There are also mixed cuts, which combine elements from both the brilliant and step cuts.
The choice of diamond cut often depends on the original shape of the rough stone, the location of internal flaws or inclusions and the preservation of carat weight.
Grading a diamond's cut is complicated, because it requires a trained eye to judge quality and standards vary from country to country.
The diamond cut grading system used by the Gemological Institute of America (GIA) grades cuts either Excellent, Very Good, Good, Fair or Poor.
Grades are assigned based on a comparison between the sample stone's cut and a cut with mathematically “ideal” proportions – so cuts closer to the “ideal” will score higher on the scale. But most of the organizations to have developed grading scales disagree on what these ideal proportions are, which makes the measurement yet more complicated.
A diamond's cut is crucial to the stone's beauty and value – but of all the four Cs, it is the most complex and difficult to analyze.
For engagement rings, round brilliant and princess cuts are the most popular, but there are a wide variety of cuts that come in and out of fashion.
The higher the cut grade a buyer chooses, the more they maximize the beauty of a diamond at its carat weight.