A great cut aims to maximise the brilliance, dispersion and fire of a diamond. Brilliance describes the total return of white light reflected from a stone and dispersion is the breaking up of white light into its component spectral colours. The visual appearance of these flashes of coloured light is also known as fire. The magnificent light pattern of a stone and sparkle or light flashes we perceive when either the stone, the light or the viewer move, is known as scintillation.
Today more than 75% of all diamonds sold are round. The modern round brilliant, sometimes also called the ‘ideal cut’, was originally conceived in the early 20th century as a successor to the ‘old cut’. Following decades of research into creating perfect cuts such as ‘Hearts & Arrows’ and modern ‘Ideal cuts’ with the help of light performance-based measurement systems and computer-aided ray tracing models which calculate and map the path of light rays through a diamond, a 99% reflection rate of light has been achieved. According to the Gemological Institute of America (GIA) which grades a diamond’s cut according to its brightness, fire and pattern, there is nonetheless no perfect cut since the best proportions for dispersion never perfectly coincide with those ideally suited for maximum brilliance. Rather various different sets of angles and proportions perform well as light is never static and a stone’s reflection always lives from its interaction with light under movement.
The cut quality or make of a round brilliant includes the proportions, which is the relationship between the table, the crown, the girdle and the pavilion of a polished diamond, as well as the finish which encompasses both symmetry and polish. Symmetry encompasses a harmonious shape or even outline of a diamond as well as an orderly alignment, placement and exactness of the facets on a cut diamond. Polish strictly refers to surface quality. These three major factors ultimately determine the overall quality of a cut.
A round brilliant of perfect proportions possesses a table facet – the single large top facet – of approximately 55% of the total girdle diameter. The crown which is the upper part of the stone should feature a height of around 15% of total girdle diameter at an angle of 34 degrees (beware crown angles of greater than 40% will intensify any colour!) relative to the girdle plane. The crown is made up of the table (facet), the star facets, the bezel (kite) facets and upper girdle facets. It is separated by the stone’s middle section, called the girdle which should be no more than 3% of diameter thickness, from the bottom part of a diamond known as ‘pavilion’.The girdle prevents chipping and provides grip and a safe edge for setting a diamond. Most of the weight is also concentrated along its plane. The girdle may be either bruted (unpolished), faceted or rarely polished. Although the girdle may range from extremely thin to extremely thick, only a medium to slightly thick girdle is considered ideal. Extremely thick girdles add excess weight whereas extremely thin ones make a stone prone to chipping. A well-made girdle moreover should be even and never wavy. Astute polishers sometimes employ the girdle in compensatory facetting techniques aimed at weight retention known as digging and painting.
The pavilion is composed of lower girdle facets and pavilion (main) facets. The pavilion mains meet in a lowermost closed tip known as the cult. Modern cuts, as opposed to the old mine, cut no longer feature any facetted culet in order to prevent any light leakage. Any open culet in modern round brilliants hence will severely penalize the cut grade. The pavilion should be set at an angle of 41 degrees relative to the girdle. The total depth of an ideal cut diamond eventually comes to around 61% of the total girdle diameter.
The return of light in a brilliant can be visually estimated by observing its reflection pattern closely. The more white light is returned, the better. Sometimes a ‘fish-eye’, a grey circle under the table resulting from extinction (off-axis reflection) due to an overly shallow pavilion, may be observed. Contrarily if the pavilion is too steep at greater 43 degrees an angle, a dark centre known as ‘nail’s head’ becomes visible. More rarely a dark ring caused by a steep crown is seen. By comparing certain parameters such as the table percentage or crown height percentage against pavilion depth and pavilion angle conclusions about the proportions may be inferred.
The most important part of the ‘finish’ of a diamond, ‘symmetry’ comprises the table-to-culet alignment, the overall composition of facet angles and the state of the girdle. Symmetry may be graded from ‘excellent’ (no or minute variations) to ‘very good’ (minor variations), which is the present-day minimum benchmark, down to ‘poor’ (prominent variations).
An off-centre table or culet, an out-of-round outline or a tilted table are usually classed as major symmetry faults, whereas asymmetrical facets, extra facets, truncated or narrow facet corners and a non-octagonal table are seen as minor issues. Another seemingly minor fault that may actually cause the symmetry grade to collapse is known as ‘twisting’. ‘Twisting’ basically describes a misalignment of crown and pavilion facets (uneven pavilion) at the junctions of the upper bezel and lower mains in the girdle. If more than a few of these misalign, the result is a broken reflection pattern and twisted light effect.
Last comes the evaluation of ‘polish’. Polishing marks though difficult to notice are easy to judge. Polish ranges from excellent which means no marks are visible and very good featuring a few minor marks all the way to poor featuring prominent, eye-visible flaws that strongly diminish the lustre of a diamond. Given modern technology, abrasives and cutting techniques, any grade less than very good is unacceptable. Fortunately polish can usually be improved the easiest. Features such a twinning and grain lines, pits, abrasion or burn marks as well as polishing rilles and scratches may all impact the polish grade.
Most diamond shapes other than the round brilliant basically belong to two basic cutting styles. These are step cuts and brilliant cuts. Mixed cuts are hybrid style. A style always refers to the principal arrangement or orientation of facets on a stone. Step cuts have parallel arranged facets and include emerald and Asscher cut. The family of brilliant cuts includes the cushion, oval, pear, marquise and heart, radiant (barion), princess, trilliant as well as rose cut and briolettes. Their kite shaped facets are radially arranged. Currently pear, oval, radiant, cushion and emerald are the most popular fancy shapes. Marquises (except in large sizes due to their spread or for the purpose of layouts) and trilliants (shoulder stones only) remain the least desired.
Modified cuts feature traditional outlines but vary in terms of facet number, pattern and arrangement from the classic cuts.
Traditional non-modified brilliant cuts have nowadays become quite scarce since they necessitate larger rough as their rough-to-polished yield is less. In other words, their costs to produce relative to modified cuts remains higher. This imbalance in fresh supply is further aggravated by a continuously strong demand which is directly related to their beauty. Modified cuts are arguably less subtle and beautiful than classic cuts due to their more broken-up reflection pattern. This observation is indirectly supported by the fact that ‘Old Mine’ cuts have seen a remarkable revival in popularity throughout the last 20 years due to their understated elegance. Old cuts usually feature an adequate level of brightness along with relatively subdued fire imparting an air of refinement.
Another drawback is the fact that the altered angles of a modified cut considerably bend and extend the path a light ray travels intensifying any hue that is present. As a result of the colour usually falls by at least one grade compared to a non-modified cut and even two colour grades when compared to a classic round brilliant cut. However, used to their advantage modified cuts fancy cuts are best employed to reinforce colour face-up in natural fancy coloured diamonds where they have revolutionized cutting.
Fancy shapes are generally graded more leniently than round brilliants. They are assessed only for symmetry and polish with a grade for proportions not usually being assigned. Poor proportions, when instantly recognized, are nevertheless undesirable. In step cuts, such issues may appear as dark crosses whereas in fancy brilliant shapes they show up as opposing dark areas of extinction such as the ‘bowtie effect’. This happens when a stone features unusual pavilion angle variations or has been cut too shallow or too deep. The appropriate total depth range goes from 50% to 69% but varies according to the actual shape.
Symmetry probably is the single most important parameter in the evaluation of fancy cuts which are primarily graded for shape appeal. The general attractiveness of their face-up outline is paramount. However, ‘good’ paper grades may sometimes be acceptable and not necessarily preclude a stone from being pretty. Moreover, cultural or personal preferences may skew a particular choice in favour of a certain shape. Also, minor aspects of the cut such as the girdle are given slightly less importance when compared to a round brilliant. Nonetheless, there are some golden rules. These include universally accepted ideal length-to-width ratios which vary according to the specific shape. For example, an ideal oval should be approximately 1.35 times as long as its width.
Furthermore, fancy shapes should feature a culet (facet) or keel line (in the case of step cuts) that is well centred and never longer than the average width of the stone. Emerald cuts or radiant cuts should neither posses corners that are too wide or too narrow or worse unequal. Also, their facets and sides should be parallel and well aligned. Potential symmetry issues extend beyond basic misalignments, asymmetrical or mishappened facets. A buyer should also be attentive that the points on a heart, pear or marquise are both well-centred and well-defined. Rounded points are undesirable. Same is true for misshapen lobes on heart shapes. A beautifully sculpted heart should also feature a well-defined cleft of appropriate length. Uneven or high shoulders (in ovals and pear shapes), bulging, flat or unequal wings (marquise, pear) and a flat head (pear) as well as uneven belly (oval & pear) are all symmetry flaws commonly seen in fancy cut diamonds.