Diamond cut, one of the 4 C's determining diamond quality, refers to how well a diamond's facets interact with light. Often confused with shape (the external outline), cut actually determines a diamond's ability to transmit light and sparkle. Precise craftsmanship is crucial to achieve proportions, symmetry, and polish that maximize light return. A well-cut diamond will display superior sparkle and brilliance.

Most modern white diamonds come with a Gemological Institute of America (GIA) certificate. While the GIA is highly respected, their cut grading system has limitations. Notably, they only provide cut grades for round diamonds, using a scale from Excellent to Poor, with separate grades for polish and symmetry. It's important to understand that polish and symmetry grades don't fully represent overall cut quality.

To get a comprehensive view of a diamond's cut, it's advisable to look beyond the GIA grade. Tools like Idealscope and ASET (Angular Spectrum Evaluation Tool) images provide valuable insights into a diamond's light performance, measuring brightness and contrast. These can offer a more complete picture of a diamond's overall beauty.

Other grading systems, such as that of the American Gem Society (AGS), incorporate ASET analysis into their cut grades. However, as the AGS Grading Laboratory has recently closed and their grading certificates are less common, buyers often need to seek additional cut analysis. Various software tools can analyse cut quality using ASET, Idealscope, and other advanced imaging techniques.

Recently, GIA acquired AGS, allowing customers to request an AGS cut grade for GIA-certified diamonds for an additional fee. This option provides access to a more comprehensive cut analysis for those seeking detailed information.

Polish refers to the diamond's external surface condition after polishing. It's graded from Excellent to Poor on certificates. The difference between "very good" and "excellent" polish is only visible under a microscope.

A diamond's proportions are determined by the relationship between crown and pavilion angles and other main facet measurements. Proportions significantly influence a diamond's sparkle and face-up appearance.

Light performance, including brilliance (white light return), fire (dispersion of light into colors), and scintillation (flashes of light and dark), plays a critical role in a diamond's beauty. A GIA Excellent cut grade indicates good potential for light performance, but it doesn't measure it directly. Tools like the ASET (Angular Spectrum Evaluation Tool) and Idealscope can provide more detailed insights into a diamond's light performance.

Fire is the scattering of white light into the colors of the rainbow. Diamonds are excellent at scattering light and displaying it as a full spectrum of colors that glint sparks of red, orange, yellow, green, or blue.  

Diamonds must have good proportions for light to disperse in this manner. Imprecise, poorer cuts do not refract or scatter light as efficiently; instead, they allow the white light to escape before it can disperse a rainbow of sparkles. Flat stones have less fire and stones with higher or steeper crown heights have more fire as it gives an angle for white light that is coming out of the crown to disperse into those spectral colors. Typically high crown small table old cuts will have the most fire.

This is incredibly important and frequently overlooked. Each cut style has an ideal depth range; diamonds cut shallower or deeper than that range will suffer from lack of brightness, dark areas or see through areas. It's also important to note most fancy cut diamonds are not cut to this ideal depth range as the cutters will get a higher carat weight and value by leaving extra weight on the diamond.For example they might cut a 1ct in carat weight but only have the spread of a .90 carat diamond. This would make the 1ct stone higher in price than an ideally cut .90ct of the same shape but it would be less beautiful than the well cut smaller stone.