The finest of Diamonds are wasted if its cut is poor. A cut has two meanings, it describes the gemstone shape that it is cut to and its own cut quality.

The Gemological Institute of America created the 4 C’s within its Diamond grading system. Keep in mind that the system for the 5 C’s & 2 C’s are dealt separately as individual factors.

Determining the cut of a gemstone is very straight-forward. With the most common shapes being Round Brilliant, Marquise, Princess Cut, Pear, Oval, Step-cut, Emerald and Fancy shape. With Round Brilliant being the most popular shape, the used cut will depend on contemporary fashion trends.

It is also dependant by the rough, uncut diamond and what the desired aim is: Maximise weight, cut away defects or inclusions within the raw stone etc...

Cut quality is well known to be the most important factor – for the diamond's beauty & value. Differences in the value may vary by 50% depending on its quality of cut. As well as being the qualities of the 4 C’s most strenuous to inspect & analyse.  

To inspect & define a Diamonds cut and grade, the fractions of the stone's facets are taken into consideration to evaluate the Diamonds appearance when observed face up. This permits the examiner to be able to calculate how light interacts with the diamond itself & how it impacts its important factors which are as such:

  • Brightness: The internal & external reflection of white light to the Diamond or the amount of returned light by the diamond;
  • Fire: (dispersion): White light being divided into the spectral rainbow colours;
  • Scintillation: Quantity of sparkle & shine given by the diamond when the gemstone, source of light or observer moves. It also conceals the patterns of light and unlit areas;
  • Contrast: Variance between brighter and darker areas. A high contrast is preferable.

Some laboratories use digital models to assess a gemstones characteristics, while others continue traditionally through the use of equipment to measure selected optical properties, for example Sparkle, Brilliance and Intensity.

Although the more scientific analysis is only valid for the round brilliant cut diamonds. Other cuts do not follow such rigid definitions of the geometric proportions, if any, and are not evaluated in the same way. This is partially due to the need of developing new computer models which may not be suitable for rarely used cuts. Cuts that are Trademarked contain well defined proportions but do not fall under the scrutinising analysis of round brilliant cuts.

The G.I.A. as well as some other laboratories use one out of five grades for the Diamond Cut quality: 

  • Excellent
  • Very Good
  • Good
  • Fair
  • Poor

Without sophisticated digital models or laboratory apparatus, here are some tips to judge a diamond's visual presentation:

Brightness

Strongly dispersed light above diamonds works best to examine the gemstone's brightness. Since the stone reflects its environment its best to wear a white shirt and have no other objects that may distract nearby. The background against on which the diamond is being observed should offer a small portion of contrast; the G.I.A. recommends a grey background.

Pattern

The same dispersed light source for judging its brightness is recommended. For the very best effect, something dark and shady should be in the vicinity of the diamond to boost the pattern effect. Bright areas occur from internal & surface reflections whereas darker areas occur due to the result from light leakage or reflections of darker objects. You should consider the following:

  • Good contrast between brighter & darker areas;
  • A well balanced & symmetrical pattern;
  • Lack of distracting dark areas;
  • Is the pattern desirable.

Fire

For evaluating the fire, the above divided light sources are less acceptable while sun light is most preferable, a pen or spotlight or even incandescent light bulbs are acceptable. Ideally, an assessment should be carried out in a dark room to improve the observation of the effect. Stones that are below half a carat can be difficult to assess visually, but its possible to make a prediction of the stone's potential fire by knowing its percentage of table facet.