In GIA's five-tier cut-grade system for standard round-brilliant diamonds, Good occupies the middle position, ranking below Excellent and Very Good and above Fair and Poor. A diamond assigned this grade by the Gemological Institute of America meets a defined set of proportion, symmetry, and polish parameters that produce acceptable brilliance and fire, but with measurable compromises in light return relative to the two grades above it. The Good grade is not a mark of failure — it indicates a stone that faces up reasonably well to the unaided eye — yet it does reflect specific optical inefficiencies that trained observers and instruments can detect, and that the market prices accordingly.

The GIA Cut-Grade Framework

GIA introduced its cut-grade system for round brilliants in 2006, following more than fifteen years of research into how facet geometry, proportions, and finish interact to govern light behaviour. The five grades — Excellent, Very Good, Good, Fair, and Poor — are determined by evaluating a cluster of parameters: table percentage, crown angle, pavilion angle, girdle thickness, culet size, total depth percentage, and the quality of polish and symmetry. No single measurement alone determines the grade; rather, GIA's proprietary model assesses the combination of these variables and their collective effect on brightness, fire, scintillation, and the absence of unwanted dark areas or "fish-eye" and "nail-head" effects.

The Good grade encompasses a relatively broad range of proportion combinations. A stone may earn a Good grade because its pavilion is slightly too deep, causing light to leak through the base, or because its crown angle is too shallow, reducing the dispersion of white light into spectral colours. Conversely, an excessively shallow pavilion can produce a washed-out, glassy appearance. Thick girdles — which add weight without contributing to face-up appearance — are another common contributor to a Good rather than Very Good grade. Misaligned or unevenly sized facets, captured under the symmetry assessment, can also push a stone into this tier.

Optical Consequences

The practical difference between a Good-cut and a Very Good-cut diamond is not always obvious to a casual observer in a retail environment, particularly under the intense, directional lighting typical of jewellery showcases. However, under more varied or diffuse lighting — the conditions in which most jewellery is actually worn — the distinction becomes more apparent. A Good-cut diamond will typically show darker zones across its table or near the girdle, reflecting light back to the eye less efficiently than a well-optimised stone. Fire, the dispersion of light into spectral flashes, may be reduced if crown angles are outside the optimal range. Scintillation — the pattern of light and dark as the stone or viewer moves — may appear less crisp or balanced.

It is worth noting that optical performance within the Good grade is not uniform. A stone at the upper boundary of Good may be visually indistinguishable from a lower Very Good stone in most viewing conditions, while a stone at the lower boundary may approach Fair in its light return. Buyers evaluating Good-cut diamonds are therefore well advised to assess individual stones rather than relying on the grade alone.

Proportion Ranges Commonly Associated with Good

While GIA does not publish a simple table of cut-grade boundaries — the model is multivariable — the following proportion tendencies are commonly associated with the Good tier in round brilliants:

• Table percentage: roughly 58–65% or above 66%, depending on accompanying crown and pavilion angles
• Total depth percentage: approximately 59–62.9% or above 64.5%
• Crown angle: outside the approximately 31.5–36.5° range preferred for Very Good and Excellent
• Pavilion angle: outside the approximately 40.6–41.8° sweet spot, trending shallower or deeper
• Girdle thickness: thick to very thick, adding depth percentage without optical benefit
• Symmetry and polish: graded Good by GIA, meaning minor deviations visible under 10× magnification

These ranges are indicative rather than definitive; the interaction between variables means a stone with one parameter outside the ideal range may still achieve Very Good if other parameters compensate.

Market Position and Pricing

Good-cut diamonds occupy a distinct commercial niche. They are typically priced at a discount of roughly 10–20% relative to comparable Very Good-cut stones of the same colour, clarity, and carat weight, though market conditions and individual stone characteristics can widen or narrow this spread. This discount reflects reduced demand from buyers who prioritise light performance, and it makes Good-cut diamonds attractive to consumers for whom budget is the primary consideration, or for whom the stone will be set in a style — a bezel, a pavé surround, or a deeply shouldered mounting — where face-up brilliance is partially obscured in any case.

In the trade, Good-cut stones are also sometimes selected for use in jewellery where the diamond serves a secondary decorative role rather than as a centrepiece, or where the setting itself is the primary aesthetic statement. Some buyers also seek Good-cut stones of larger carat weight, accepting a modest reduction in light performance in exchange for greater apparent size at a given price point.

It should be noted that the Good cut grade applies specifically to GIA-graded standard round brilliants. Fancy-shape diamonds — ovals, cushions, pears, marquises, emerald cuts, and others — are not assigned a cut grade by GIA on their laboratory reports, as no equivalent standardised model exists for those outlines. Other laboratories may use different nomenclature or different grade boundaries, and direct comparisons between, for example, a GIA Good and an AGS or IGI equivalent require care.

Good Cut in Context: What It Is Not

A common misconception is that a Good cut grade indicates a poorly cut stone. This is inaccurate. GIA's Fair and Poor grades exist precisely to capture diamonds with more serious optical deficiencies — stones that leak substantial light, display pronounced dark centres, or exhibit severe symmetry deviations. A Good-cut diamond, by contrast, is a commercially viable, aesthetically acceptable stone that simply does not achieve the optimised light return of the upper two tiers. For buyers who have examined the stone in person and are satisfied with its appearance, the grade need not be a disqualifying factor.

What the grade does signal is that the stone should be evaluated individually and critically, and that comparisons with Very Good or Excellent stones of similar grading parameters are worthwhile before a purchase decision is made. The grade is most usefully understood not as a quality verdict but as a prompt for closer examination.

Further Reading

• GIA: Diamond Cut — gia.edu
• Gems & Gemology: Grading the Round Brilliant Cut Diamond (GIA, 2004)