AGS-1: The American Gem Society's Second-Highest Cut Grade
AGS-1: The American Gem Society's Second-Highest Cut Grade
Very Good light performance on the AGS 0–10 cut-quality scale
AGS-1 is the second-highest cut-quality designation on the American Gem Society (AGS) diamond grading scale, a numerical system running from 0 (Ideal) to 10 (Poor). A diamond graded AGS-1 demonstrates very good light performance — strong brilliance, fire, and scintillation — with proportions or finish characteristics that fall just outside the narrow tolerances required for the AGS 0 Ideal designation. The grade is issued by the American Gem Society Laboratories (AGSL), which pioneered light-performance-based cut grading and remains one of the two most widely recognised diamond grading authorities in the United States alongside the Gemological Institute of America.
The AGS Grading Scale in Context
The AGS numerical scale assigns whole-number grades from 0 to 10 across three principal components: cut (encompassing proportions and light performance), polish, and symmetry. The overall cut grade on an AGSL Diamond Quality Document is determined by the lowest of these component scores, meaning a stone with exceptional proportions but a single polish grade of AGS-1 will carry an overall grade of AGS-1 rather than AGS 0. This integrated approach differs from the GIA system, which reports cut, polish, and symmetry as separate descriptive grades (Excellent, Very Good, Good, and so forth) without collapsing them into a single numerical score.
AGSL's cut-grading methodology is underpinned by ray-tracing and light-performance modelling, most notably its Angular Spectrum Evaluation Tool (ASET), which maps how a diamond captures and returns light from different angular zones. An AGS-1 stone will show measurable, high-quality light return under this analysis, but with minor leakage or contrast patterns that prevent it from meeting the strict AGS 0 threshold.
What Distinguishes AGS-1 from AGS 0
The AGS 0 Ideal grade is defined by tightly bounded proportion ranges — table percentage, crown angle, pavilion angle, total depth, and girdle thickness — that collectively optimise the balance of brilliance and dispersion for a round brilliant diamond. AGS-1 proportions may deviate modestly from these ranges: a crown angle slightly steeper or shallower than ideal, a table fractionally outside the preferred window, or a girdle thickness at the upper boundary of acceptable. In practical terms, the visual difference between a well-cut AGS-1 and an AGS 0 stone is often imperceptible to the unaided eye under normal viewing conditions; the distinction is primarily one of measurable optical modelling rather than obvious face-up appearance.
Polish and symmetry grades of AGS-1 correspond broadly to what GIA would describe as Very Good, indicating minor surface characteristics or facet-alignment deviations visible under 10× magnification but not detectable to the naked eye.
Market Position
In the trade, AGS-1 diamonds occupy a position analogous to GIA Excellent or Very Good cut stones, depending on the specific proportions involved. Because the AGS 0 Ideal designation carries a recognised premium — particularly among consumers who have researched light-performance grading — AGS-1 stones are sometimes perceived as a value tier, offering performance close to Ideal at a more moderate price point. Jewellers and dealers familiar with the AGSL system will often examine the underlying proportion data on an AGS-1 certificate rather than treating the grade as a monolithic category, since two stones sharing the same AGS-1 overall grade may differ meaningfully in their individual component scores and actual face-up appearance.
It should be noted that AGSL also grades fancy-shape diamonds — including princess, oval, cushion, and others — using its light-performance methodology, and AGS-1 designations exist for these shapes as well, though the proportion benchmarks differ from those applied to round brilliants.