The American Gem Society (AGS) evaluates diamond cut quality on a numeric scale running from 0 to 10, where 0 (AGS Ideal) represents the highest attainable light performance and 10 represents the poorest. Grades AGS-2 through AGS-9 occupy the remainder of that range, describing diamonds whose proportions, symmetry, polish, or combination thereof fall progressively further from the Ideal benchmark. The scale is documented and administered by the AGS Laboratories (AGSL), and its methodology — grounded in ray-tracing light-performance modelling rather than simple proportion tables — distinguishes it from the five-category verbal system used by the Gemological Institute of America (GIA).

The Scale in Context

AGS cut grades are assigned across four components — light performance, proportions, symmetry, and polish — and the overall grade is determined by whichever component scores lowest. The numeric grades translate loosely to familiar verbal descriptors:

• AGS-0: Ideal
• AGS-1: Excellent
• AGS-2: Very Good
• AGS-3 and AGS-4: Good
• AGS-5 through AGS-7: Fair
• AGS-8 through AGS-9: Poor
• AGS-10: Poor (the lowest grade)

This granularity — ten discrete steps rather than five verbal categories — allows the system to draw finer distinctions within the middle and lower tiers, though in practice the differences between adjacent grades (say, AGS-5 versus AGS-6) are rarely perceptible to the unaided eye without direct comparison.

AGS-2: Very Good

A diamond graded AGS-2 delivers strong light performance with only minor, measurable deviations from ideal proportions or finish. Such stones may exhibit very slightly reduced brightness or scintillation compared to AGS-0 and AGS-1 specimens, but the difference is subtle under most viewing conditions. AGS-2 diamonds represent a practical choice for buyers who prioritise value over the marginal performance gains of the top two grades.

AGS-3 and AGS-4: Good

The Good tier encompasses diamonds whose proportions or finish introduce more noticeable compromises in light return. Common contributors include table percentages or crown angles that fall outside the tighter ranges favoured by the AGS model, or polish and symmetry characteristics that interrupt the intended optical pattern. These stones remain commercially acceptable and are widely traded, but their light performance is measurably inferior to Ideal and Excellent grades when assessed with angular-spectrum modelling or direct photometric tools such as the ASET (Angular Spectrum Evaluation Tool).

AGS-5 through AGS-9: Fair and Poor

Grades in the Fair and Poor range reflect increasingly significant departures from optimal cut parameters. Diamonds in this tier may suffer from excessive depth or shallowness (producing the characteristic "fisheye" or "nail-head" effects), steep or flat crown angles that suppress dispersion or brilliance respectively, or finish grades that introduce visible surface irregularities. AGS-5 through AGS-9 stones are uncommon on AGS Laboratory reports for a straightforward reason: cutters and dealers who submit diamonds to AGSL typically do so because they expect strong results. Poorly cut goods are more often submitted to laboratories whose reports do not include a cut grade, or where the grading methodology is less stringent.

Practical Significance in the Trade

Because the AGS scale is performance-based rather than purely proportion-based, a diamond can achieve AGS-2 or better even with proportions that fall outside the ranges traditionally associated with "ideal" cutting, provided its actual light return — as modelled by ray-tracing — meets the threshold. Conversely, a stone with proportions that appear acceptable on paper may score lower if its symmetry or polish degrades the optical output. This makes the AGS numeric grade a more direct proxy for visual performance than a simple proportion report, and it is one reason the system is respected among trade professionals and sophisticated consumers who use light-performance imagery alongside the grade itself.

In the secondary and wholesale markets, AGS-2 stones are routinely grouped with GIA Very Good grades for pricing purposes, while AGS-3 and AGS-4 align broadly with GIA Good. Grades below AGS-5 carry meaningful price discounts relative to well-cut goods of equivalent colour and clarity, reflecting the market's consistent preference for superior light performance.

Further Reading

• AGS Laboratories — Cut Grading Methodology
• GIA — The 4Cs: Cut