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GIA vs AGS Grading Scales: Why the AGS 0 Still Matters Inside GIA

6/6/2026 · 5 min read

This is a spoke of our hub How to Read a GIA Diamond Report. If you've already read the hub, you know GIA's cut grade tops out at Excellent — a wide bucket that lumps a genuinely top-light-performance round in with a merely well-proportioned one. The reason a separate AGS scale ever mattered, and the reason it still matters inside today's GIA reports, is that Excellent and Ideal are not the same word.

The two scales, side by side

GIA cut grade (round brilliants only):

  • Excellent
  • Very Good
  • Good
  • Fair
  • Poor

GIA's grade is computed from a combination of proportions (table, depth, crown angle, pavilion angle), polish, and symmetry, with a face-up appearance assessment layered on top. It's a robust, conservative grade. The problem is that "Excellent" covers a wide proportion range — two stones can both be triple-Excellent and look meaningfully different in person.

AGS cut grade (the old AGS Laboratories scale, still alive in GIA's AGS Ideal supplement):

  • 0 (Ideal)
  • 1 (Excellent)
  • 2 (Very Good)
  • 3–4 (Good)
  • 5–7 (Fair)
  • 8–10 (Poor)

AGS graded cut on light performance — actual brightness, contrast, dispersion, and leakage measured with ray-tracing and ASET imaging — rather than just measured proportions. A stone earned a 0 only if it returned light in a specific pattern, not because its numbers fell in a bucket. That's why "AGS 0" became trade shorthand for the top tier of cut a round brilliant can reach.

What happened to AGS

AGS Laboratories closed in late 2022. GIA acquired the AGS intellectual property, the research staff, the cut-grading software, and the Las Vegas facility. The science survived the merger: it now ships as the AGS Ideal Report, a digital supplement that can be added to an eligible GIA report on qualifying round brilliants and select fancy shapes, natural and lab-grown, for roughly $25 per stone. The AGS 0 still means what it always meant — top light performance — it just gets printed alongside the GIA grade now instead of on a standalone AGS certificate.

For the inside-the-trade view on how the older AGS certificates are still being valued in the secondary market, the Antwerp dealer Ajediam's notes on certification provenance are a useful industry-side reference, particularly on how pre-2022 AGS paper is treated when a stone changes hands.

How to translate one scale into the other

There is no clean one-to-one conversion, and anyone who claims otherwise is selling you something. But the trade rule of thumb is reliable:

  • GIA Triple Excellent + AGS Ideal 0 is the genuine top tier. Roughly the top 10–15% of GIA Triple Excellent stones earn the AGS 0 supplement.
  • GIA Triple Excellent without AGS 0 is still an excellent stone — most of the market lives here — but you're inside the proportion-measurement bucket, not the light-performance one.
  • GIA Excellent cut without polish and symmetry both Excellent is not equivalent to AGS Excellent (1). It's somewhere between AGS 1 and AGS 2.
  • GIA Very Good is roughly AGS 2 to low AGS 3.

For the specific proportion ranges that earn a GIA Excellent on a round, see the hub article. The point of the AGS supplement is that two stones can both hit those proportion targets and still perform differently — that's what AGS 0 isolates.

When the AGS 0 is worth paying for

Three cases. First, rounds priced above roughly $4,000–$5,000, where the cut premium is a meaningful slice of the spend and you want the harder test to separate the top performers from the rest of the Excellent bucket. Second, any round being marketed as "super ideal" or "true hearts and arrows" — make the seller put the AGS 0 supplement on it before you accept the premium. Third, lab-grown rounds where you're paying for a top-tier visual and the IGI report doesn't carry an equivalent light-performance grade; the AGS Ideal supplement is the most defensible cut grade you can attach.

Skip the supplement on rounds under that price band — the GIA Excellent is enough — and skip it on fancy shapes that don't qualify. It's not a universal upgrade. It's a targeted one.

The one-line summary

GIA Excellent is a measurement bucket; AGS 0 is a light-performance verdict, and the difference between them is the difference between a well-proportioned round and a round that actually performs — for which a $25 supplement is the cheapest insurance on the page. For the full report walkthrough, go back to the hub article on reading a GIA report; for the fluorescence call that interacts with cut performance under daylight, see GIA fluorescence grades explained.