Diamond cut explained

When jewellers talk about cut, they mean the craftsmanship applied to a rough diamond during the cutting and polishing process. A well-cut diamond has proportions that direct light efficiently from the table through the pavilion and back out to your eye. A poorly-cut diamond loses light through the sides or base, leaving it looking dark or lifeless.

Cut is graded independently of shape. A round brilliant, oval, and emerald cut can all be Excellent or Poor cuts — the grade reflects proportions, not the outline. GIA grades round brilliants on a five-point scale: Excellent, Very Good, Good, Fair, and Poor. For fancy shapes — ovals, pears, cushions, and the rest — there is no equivalent official cut grade, which is why proportion ranges and face-up spread matter even more when choosing non-round stones.

Of the 4Cs, cut is the only one entirely within the craftsman’s control. Colour and clarity are set by nature. Carat is a measure of weight. But every degree of crown angle, every percentage point of total depth, and the quality of every polished facet is a decision made by a skilled diamond cutter. A well-cut stone is a well-made one.

GIA grades round brilliant cut diamonds on five levels: Excellent, Very Good, Good, Fair, and Poor. Each grade reflects a range of proportions, symmetry, and polish scores rather than a single set of measurements. Two Excellent-grade diamonds can have slightly different proportions and both earn that grade — what matters is that both fall within the range GIA has determined produces outstanding light performance.

Excellent and Very Good cover the territory most buyers should target. An Excellent cut represents the top tier — proportions are optimised for maximum brilliance, fire, and scintillation. Very Good stones are nearly as well-cut and often represent better value per carat. The difference between Excellent and Very Good is typically imperceptible in everyday wear.

Good and Fair grades represent meaningful compromises. Good-cut diamonds can still be attractive, but proportions are noticeably outside the ideal range and light performance suffers. Fair and Poor grades are unsuitable for engagement ring centre stones — the visual penalty is significant. At À Vie Diamonds we set Excellent as our minimum for round brilliants, and we review proportions within that grade rather than treating the grade itself as sufficient reassurance.

The common approach to diamond buying is to set a carat weight target and then work backwards. This is exactly the wrong order. A 1.00ct diamond with a mediocre cut will look smaller, duller, and less impressive than a 0.90ct diamond with an Excellent cut — and the Excellent-cut stone will often cost less per carat once you account for the premium on exact round-number weights.

Cut affects what you see. Colour and clarity affect what a trained grader can detect under magnification. Most buyers cannot tell the difference between a G and an H colour in everyday conditions. Most buyers cannot see the difference between VS1 and VS2 inclusions face-up. But every buyer can immediately tell the difference between a bright, lively diamond and a flat, dull one — and that difference is cut.

The practical approach: fix Excellent cut as a non-negotiable, then choose the highest carat weight that stays within your budget at G–H colour and VS2–SI1 clarity. This order of priorities consistently delivers the best-looking stone for a given spend.

A GIA grading report for a round brilliant diamond includes a cut grade near the top of the proportions section. Below it you will find individual scores for polish and symmetry, each graded on the same five-point scale. A certificate that reads Excellent cut, Excellent polish, Excellent symmetry — sometimes referred to as Triple Excellent or 3EX — indicates a stone that has been finished to the highest standard at every stage of the cutting process.

Below the grades are the actual proportions: table percentage, total depth percentage, crown angle, pavilion angle, girdle description, and culet size. These numbers tell you more than the grade alone. Two Excellent-cut diamonds can have different proportions — one with a 34.5° crown angle and 41.0° pavilion may handle light slightly differently to one with a 35.5° crown and 40.8° pavilion, even though both earn Excellent. The ideal ranges are: table 54–57%, total depth 59–63%, crown angle 34–35°, pavilion angle 40.6–41.8°.

For fancy shapes — oval, pear, cushion, emerald, and others — there is no GIA cut grade. For these stones, proportions and face-up spread are the only way to assess cut quality. We always provide millimetre measurements and review imagery for fancy-shape stones before adding them to a shortlist.