Cut Grade of Diamonds

The cleanest diamond is wasted if the cut of it is poor. Often, "cut" is thought of the shape the gemstone is cut but it also describes the cut quality. The GIA (Gemological Institute of America) developed the "4Cs" system and in its diamond grading system, Cut describes both shape and cut quality. In the extension of the 4Cs – the 5Cs and 2Ts – those two factors are dealt with separately.

Determining the shape of the stone is straight-forward. The most commonly used shapes are Round brilliant, marquise, princess cut, pear shape, oval, step-cut, emerald cut and fancy shape, to name a view. Round brilliant is by far the most commonly used cutting style but what cut is used depends on current fashion trends, kind of jewellery, personal preference and so on. It also depends on the rough, uncut diamond itself and what the goal is: Maximise weight, cut away defects or inclusions in the rough stone, etc.

Cut quality is a very important factor – if not the most important one – for the diamond's beauty and price tag. Difference in the stones value can vary up to 50% depending on the cut quality. Interestingly, it is also the most difficult aspects of the 4Cs to analyse.

To analyse and grade a diamond's cut, proportions of the stone's facets are taken into account to assess the appearance of the diamond when viewed face up. This allows to calculate how light interacts with the diamond and how it impacts on important factors such as:

  • Brightness: How white light is reflected internally and externally from a diamond, or the amount of light returned by a diamond;
  • Fire (dispersion): White light being split into the spectral colours (rainbow colours);
  • Scintillation: Amount of sparkle produced by the diamond when the stone, light source or observer moves. It also covers patterns of light and dark areas;
  • Contrast: Difference between bright and dark areas. A high contrast is desirable.

Some laboratories are using computer models to assess a diamond's optical characteristics, while others do use equipment to measure some selected optical properties, for example Brilliance, Sparkle and Intensity.

This rather scientific analysis is only valid for round brilliant cut diamonds. Other cuts do not have such stringent definitions of the proportions, if any, and are not assessed in the same way. Partially this is due to the necessity of developing new computer models which may not pay off for some rarely used cuts. Trademarked cuts do have well defined cutting proportions but do not fall under the scrutinising analysis of round brilliant cuts.

The GIA as well as some other laboratories are using one out of five grades for the cut quality of a diamond:

  • Excellent
  • Very Good
  • Good
  • Fair
  • Poor

Without sophisticated computer models or laboratory equipment, here are some tips to evaluate a diamond's visual performance:

Brightness
Strongly diffused light above the diamond works best to assess the stone's brightness. Since a diamond reflects its surroundings, it is best to wear a white shirt and have no other distracting objects nearby. The background against which the diamond is observed should provide some contrast; the GIA recommends a gray background.

Pattern
The same diffused light source as for evaluating brightness is recommended. For best effect, something dark should be in the vicinity of the diamond to maximise the pattern effect. Bright areas come from internal and surface reflections whereas dark areas are the result from light leakage or reflections of dark objects. You should consider the following:

  • Good contrast between bright and dark areas;
  • A well balanced and symmetrical pattern;
  • Lack of distracting dark areas;
  • Do you like the pattern?

Fire
For evaluating fire, the above diffused light sources are less suitable and you should use sunlight, a pen or spotlight or even incandescent light bulbs. Ideally, assessment should be carried out in a dark or slightly dark room to improve the observability of the effect. Stones under half a carat are difficult to assess visually, but it is possible to make a prediction of the stone's potential fire knowing its table facet percentage.

The above is only a quick introduction into a complex subject and we hope it gives you a good starting point when it comes to assess the cut quality of a diamond.

Further Reading

This article was a short introduction of the complex subject of grading the cut style and quality of a diamond. Below, you find a list that covers further reading material. Go back to read about the famous 4Cs or learn more about the other 3 Cs by clicking on the relevant links:

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