Cut is commonly confused with shape. The true meaning of cut in a diamond grading context is the various proportions, symmetry and finish of the diamond. What makes evaluation of cut so difficult is that there is more than one way, and many opinions, as to how to cut a diamond to optimize its optical properties and hence maximize its brilliance and beauty
Proportion refers to the angles and relative measurements of a polished diamond. More than any other feature, proportions determine a diamond's optical properties. Scientific studies have shown that table size, crown angle, and pavilion depth have a dramatic effect on a diamond's appearance.
Given this, cutting a diamond to produce the greatest possible return of light to the observer depends on the interrelationship between these three components. There isn't however just one specific combination of these three that will yield the most brilliant diamond. Various combinations of these proportions can potentially produce diamonds of equal brilliance and beauty.
Girdle Diameter Table Size Crown Angle
The diagram on the right only looks at pavilion depth (distance from the bottom of the girdle to the culet), one of the 3 major proportions affecting a diamond's brilliance. A pavilion depth that is too shallow or too deep will allow light to escape from the side of the stone, or through the bottom, causing a dark and dull appearance and reducing the diamond's brilliance. A well-cut diamond will direct more light through the crown thus maximising the brilliance of the diamond. Note that the diagram is an oversimplified 2-dimensional representation of the complex behaviour of light passing through the diamond which in reality includes refraction, dispersion and reflection.
WIDTH OF THE TABLE HEIGHT OF THE CROWN DEPTH OF THE PAVILION
Symmetry refers to the exactness of correspondence and alignment between facets of a diamond. Problems that diminish the symmetry of a diamond, usually due to poor or careless workmanship, are: off-centre table, off-centre culet, non-round or wavy girdle, non-parallel girdle, non-parallel table, table not a regular octagon, misaligned facets, misshaped facets, naturals and extra facets not graded under clarity. Poor symmetry can cause light to be misdirected as it travels into, through and out of the diamond.
Finish refers to the precision of the cut, the condition of the girdle and the diamond's polish. The polish grade given to a diamond describes the smoothness of the facets. Poor polishing can dull the surface of the facet creating a dull sparkle. The diagram on the right outlines the various facets of a round brilliant cut diamond. Often the culet is left pointed and hence is not a facet, giving a total of 57 facets. The girdle is sometimes faceted but these facets are not counted in the normal way.
Hearts & Arrows is an optical phenomenon which can arise when a round brilliant cut diamond has been well cut. The diagram in the right margin illustrates the phenomenon which can be viewed with a symmetry viewer.
Well cut diamonds cost more for 3 reasons. The first is that they look the most beautiful and hence are in greater demand. Secondly more skill was used and time was expended by the diamond cutter to get all the proportions and angles to fall within a desired range of values. Thirdly, and probably most importantly, a larger stone is required to cut a well-cut diamond than a poorly cut diamond of the same carat weight. ie you lose more of the diamond in the cutting process because the cutter concentrates not on getting the heaviest finished diamond out of the rough diamond, but rather the most brilliant diamond.
As an example, a well cut 1 carat diamond has a diameter of approx 6.5mm whereas a shallow cut 1 carat diamond will have a diameter greater than this and so appear larger when looked at from the top. This isn't as good as it sounds because you will be sacrificing brilliance of the diamond for apparent size. These badly cut diamonds often have a dull lifeless look known as "fisheye". Conversely a deep cut 1 carat diamond will have a diameter less than 6.5mm so you will be paying for a diamond that not only looks smaller than it should, but also doesn't have the brilliance of a well cut stone. These stones often have a dark dull area in the centre which is a phenomenon known as "nailhead".
In conclusion, it is very important to consider only purchasing a well-cut diamond even if this is to the detriment of the other 3 C's. The better cut will make up for the lower clarity, colour and possibly size (carats) in brilliance and beauty. Indeed bigger is not always better. A diamond can have excellent colour, clarity and size, but if cut badly will lack brilliance and beauty.
There are many terms used to indicate how well a diamond is cut. For round brilliant cut diamonds look for a "Cut" or "Proportions" grade of Excellent, sometimes called Ideal. Very Good and Good cut grades offer excellent value for money. Below these are Fair, Poor or Unspecified, be weary of these unless of course that is what you are after and you are not being charged a premium. NB that the above grades are usually only provided on certificates for Round Brilliant Cut stones. For fancy shapes you will have to rely on your own knowledge and the trustworthiness of the supplier.