If you’ve ever bought a piece of diamond jewellery, you will probably have heard jewellers talking about diamond cuts. Many people assume that the cut of a precious stone refers to its shape (for example, round or emerald), but this is not the case. The cut actually concerns how well the flat faces of a diamond - known as facets - reflect light. There are many types of diamond cuts available, but the quality of each can be assessed by considering the following features:
The world-renowned gemstone research body the Gemological Institute of America (GIA) grades the cut of a diamond by calculating the proportions of the facets that affect how a diamond looks face-up. These measurements are used to determine how well a diamond handles light.
A well-cut diamond interacts with light to create stunning visual effects. White light is reflected, resulting in a magnificent brightness, and is dispersed to appear as flashes of colour. The scintillation of a diamond refers to intense white and coloured sparkles of light that appear when a diamond, its platform or the viewer is in motion. An expertly-cut diamond will have very few ‘dead’ areas, or areas where no flickers of light are seen.
To grade different diamond cuts, the weight ratio of the jewel needs to be considered. The weight ratio refers to the weight of the stone relative to its diameter and it is used to determine the ‘hidden weight’ of a diamond. A diamond that has been cut deeply will have a greater proportion of its weight hidden lower down. This means that its diameter will be smaller than a diamond that has been cut well.
The thickness of the outer edge, or girdle, of a diamond is also taken into account when determining the grade of the cut. A thicker girdle will add weight (and therefore value) to a diamond. However, it will not appear any bigger when viewed from above as the majority of its weight is hidden in the depths. A jewel with a thick girdle is less prone to chipping.
The facets of a perfect diamond are completely symmetrical. Symmetry can be examined by viewing a diamond using 10x magnification. A diamond ranked ‘excellent’ in regard to its symmetry will have no visible defects under magnification. A ‘poor’ diamond, meanwhile, will have symmetry flaws that can be identified with the naked eye. Poor symmetry results in light being reflected badly, decreasing the diamond’s overall brilliance.
The polish of a diamond relates to how smooth its surfaces are. Microscopic defects on the facets of a diamond can affect light patterns and diminish the overall quality of the cut. Blemishes can sometimes be caused by the polishing process if the wheel scrapes tiny crystals across the face. A diamond rated ‘excellent’ in terms of its polish will have no or very few minor polish flaws and these features will only be visible under high magnification. A ‘poor’ diamond, however, will have major blemishes that can be easily identified.