Excellent Cut
Excellent Cut
The highest cut grade in GIA's grading system for round brilliant diamonds
An Excellent cut is the highest grade on the five-point cut-grading scale established by the Gemological Institute of America (GIA) for round brilliant diamonds, the other grades being Very Good, Good, Fair, and Poor. A diamond earning this designation has been fashioned to proportions, symmetry, and polish that together produce optimal interaction with light — maximising brilliance (the return of white light to the eye), fire (the dispersion of light into spectral colours), and scintillation (the pattern of light and dark as the stone or observer moves). Representing roughly the top three per cent of all polished round brilliant diamonds submitted to GIA, the Excellent grade is the benchmark against which serious buyers and dealers measure cutting quality.
How the Grade Is Determined
GIA's cut grade for round brilliants is not a single measurement but a holistic assessment integrating seven components: brightness, fire, scintillation, weight ratio, durability, polish, and symmetry. Each of these is evaluated individually, and the final cut grade reflects the combination of all seven. A stone that excels in one component but is deficient in another — for instance, a well-proportioned diamond with poor polish — will not achieve an Excellent overall grade.
Proportion parameters that typically characterise an Excellent-cut round brilliant include:
- Table percentage: approximately 53–58% of girdle diameter
- Crown angle: approximately 34–35°
- Pavilion angle: approximately 40.6–41°
- Total depth percentage: approximately 59–62.5%
- Girdle thickness: thin to slightly thick, uniform
- Culet: none to very small
These ranges are not absolute thresholds. GIA's research, published across multiple issues of Gems & Gemology, demonstrated that no single set of proportions defines optimal light performance; rather, a range of proportion combinations can interact to produce equivalent face-up appearance. A diamond with a table of 56% and a crown angle of 35° may perform identically to one with a table of 54% and a crown angle of 34.5°, provided the pavilion angle and other parameters are appropriately matched.
Polish and Symmetry Within the Grade
GIA grades polish and symmetry on their own five-point scales (Excellent, Very Good, Good, Fair, Poor) and reports them separately on the grading report. A diamond may receive an overall Excellent cut grade while carrying individual polish or symmetry grades of Very Good rather than Excellent, because the cut grade is a composite assessment. However, a stone with Excellent grades across all three — cut, polish, and symmetry — is colloquially described in the trade as a triple Excellent, or triple EX, and commands a meaningful premium in most markets, particularly in East Asia, where triple-EX stones are strongly preferred by consumers.
Polish grades reflect the quality of the facet surfaces as finished by the polishing wheel: an Excellent polish means no polish-related features (such as burns, scratches, or abrasions) are visible under ten-times magnification. Symmetry grades reflect the precision of facet placement, shape, and alignment: an Excellent symmetry means no deviations — off-centre tables, wavy girdles, or misaligned facets — are detectable at ten-times magnification.
Scope and Limitations of the Grade
It is essential to understand that GIA's cut grade applies exclusively to standard round brilliant diamonds. Fancy-shape diamonds — ovals, cushions, pears, marquises, emerald cuts, radiant cuts, and all other non-round outlines — do not receive a GIA cut grade on their laboratory reports. This is not an oversight but a reflection of the mathematical complexity involved: the round brilliant has a single, well-defined ideal geometry against which performance can be modelled, whereas fancy shapes involve subjective length-to-width preferences and variable faceting architectures that resist a single grading standard. Buyers evaluating fancy shapes must therefore rely on individual proportion measurements, visual inspection, and, where available, light-performance imaging technologies such as ASET (Angular Spectrum Evaluation Tool) or Idealscope imagery.
The grade also applies only to diamonds. Coloured gemstones — sapphires, rubies, emeralds, and all other species — are not graded on any equivalent standardised cut scale by GIA or any other major laboratory. Cut quality in coloured stones is assessed descriptively, with attention to windowing, extinction, and overall proportions, but no numerical or categorical cut grade is issued.
Market Value and the Excellent Premium
Within the round brilliant diamond market, cut grade is widely regarded as the most consequential of the four Cs for face-up visual impact, because a well-cut stone of moderate colour or clarity will often outperform a poorly cut stone of superior colour and clarity in terms of perceived brilliance. The price differential between an Excellent cut and a Very Good cut in otherwise identical stones — same carat weight, colour grade, and clarity grade — is typically in the range of ten to twenty per cent, though this varies with market conditions, stone size, and the specific combination of colour and clarity grades involved. At higher colour and clarity combinations (D–F colour, IF–VS1 clarity), buyers tend to be more exacting, and the premium for Excellent cut is more consistently enforced.
The triple-Excellent premium is most pronounced in Japanese and Chinese retail markets, where consumer education around cut grading has been particularly thorough and where triple-EX certification is sometimes a near-mandatory requirement for bridal jewellery. In Western markets, the premium exists but is somewhat less rigid, with some buyers accepting Very Good symmetry alongside Excellent cut and polish without significant price concession.
Relationship to the "Ideal" Cut Concept
The term ideal cut predates GIA's formalised cut-grading system and derives from the mathematical work of Marcel Tolkowsky, whose 1919 treatise Diamond Design proposed a specific set of proportions — a 53% table, 59.3% total depth, 34.5° crown angle, and 40.75° pavilion angle — as optimal for a round brilliant. Tolkowsky's proportions remain influential, and some vendors and independent grading systems (notably the American Gem Society's AGS Laboratories, which uses a 0–10 scale with 0 representing ideal) use the term "ideal" to denote the highest tier of cutting quality.
GIA deliberately chose the term "Excellent" rather than "ideal" to avoid implying that a single set of proportions represents the sole path to superior light performance — a position supported by GIA's own computer modelling and face-up appearance research. In practice, a GIA Excellent cut and an AGS 0 (Ideal) grade overlap substantially, though they are not identical designations and are produced by different grading methodologies. Stones graded Excellent by GIA will generally, though not invariably, also achieve AGS 0 or AGS 1 grades when submitted to AGS Laboratories.
Practical Guidance for Buyers
Because the Excellent grade encompasses a range of proportion combinations rather than a single ideal, two diamonds both graded Excellent may differ perceptibly in face-up appearance. Buyers seeking the most visually compelling stone within the Excellent tier are well advised to examine stones in person under varied lighting conditions — diffuse overhead lighting, direct spotlighting, and natural daylight — and to request light-performance imagery where available. The Hearts and Arrows pattern, visible through a dedicated viewer, indicates a high degree of optical symmetry and is a useful secondary indicator of cutting precision within the Excellent grade, though its presence alone does not guarantee superior performance if other proportion parameters are suboptimal.