· 6 min read
Diamond Cut Grades Explained: What Sparkle Costs
Diamond cut grades explained by a jeweller: what Excellent, Very Good and Ideal really mean, which proportions matter, and where the extra money is wasted.
A diamond's cut grade rates how well the stone's proportions turn light into sparkle. It has nothing to do with shape, and nothing to do with whether the stone is natural or lab-grown. GIA uses five grades on round brilliants (Excellent, Very Good, Good, Fair, Poor), IGI adds Ideal at the top and applies its scale to some fancy shapes as well, and the old AGS 0-to-10 scale still turns up on stones certified before 2023.
What a cut grade is actually measuring
A cut grade is a summary of several separate judgements about the finished stone:
- Brightness. White light returned to your eye through the top of the diamond.
- Fire. Light split into coloured flashes.
- Scintillation. The pattern of bright and dark as the stone moves.
- Weight ratio. Whether the cutter kept extra weight in the belly of the stone to land on a round number instead of cutting for beauty.
- Durability. Very thin girdles chip, and a claw setting is unforgiving about it.
- Polish and symmetry. Graded and printed separately, but they feed into the overall cut grade on a GIA report.
The weight ratio point is the one most people miss. A 1.00 ct stone cut deep to protect its weight will look smaller face-up than a well cut 0.92 ct, and you pay for those extra points regardless. That is the single most common reason a diamond disappoints in person after looking fine on paper.
The scales are not interchangeable
- GIA grades cut only on round brilliants: Excellent, Very Good, Good, Fair, Poor.
- IGI runs Ideal, Excellent, Very Good, Good, Fair, Poor, and does publish cut grades for several fancy shapes.
- AGS used 0 (Ideal) through 10. The lab closed in 2022 and was absorbed by GIA, so you will only see this on older paper. It was a strict, well-respected scale and an AGS 0 is still a good sign.
Most lab-grown stones on the market carry IGI reports, and most natural stones carry GIA. Grading is human judgement, and IGI is generally read in the trade as a shade more generous than GIA. An IGI Excellent is not automatically a GIA Excellent. That does not make IGI paper bad, it just means you should not compare two reports from different labs as though the words mean exactly the same thing.
The numbers behind the grade
For a round brilliant, the grade comes out of a fairly narrow band of proportions.
Table and depth
Excellent territory is roughly a 53 to 58 per cent table with a total depth of 59 to 62.5 per cent. Outside that, light starts leaking out of the bottom or the pavilion acts like a mirror and the stone goes dark under the table.
Crown and pavilion angles
Pavilion angle is the number that does the most work: about 40.6 to 41.0 degrees. Half a degree of drift there costs you visible light return. Crown angle sits around 34 to 35 degrees and controls how much fire you get versus flat white brightness.
Girdle and culet
Thin to slightly thick is what you want. "Extremely thin" is a real chipping risk on a shape with a point, such as a pear or marquise. The culet should read "none" or "very small"; anything larger shows as a grey dot when you look straight down.
Reading a report quickly:
- Check the shape. If it is not a round brilliant on a GIA report, there is no cut grade to read.
- Read the cut grade, then the polish and symmetry lines underneath it.
- Check the table and depth percentages against the ranges above.
- Check the girdle description for "extremely thin".
- Look at the measurements. On a round, a 1.00 ct stone should be around 6.4 mm across. Noticeably smaller means weight is hiding underneath.
Fancy shapes have no GIA cut grade at all
This surprises people. On an oval, pear, emerald, cushion or radiant, a GIA report gives you polish and symmetry and nothing else. There is no official verdict on how the stone performs. Two ovals with identical carat, colour and clarity can look completely different, and the report will not tell you which is which.
So for fancy shapes you judge by eye and by measurements. Look at the length-to-width ratio, which is a matter of taste rather than quality, and look for the bow-tie: a dark band across the middle of elongated shapes. Every oval has one to some degree. A faint one is normal, a heavy black one is a badly cut stone being sold on carat weight. This is why we show every stone in our loose diamond search in 3D rather than as a stock illustration. On fancy shapes it is the only honest way to shop.
What each grade actually costs
Rough figures for a 1.00 ct round in the G/VS2 range, holding everything else equal:
| Cut grade | What you see | Typical price effect | Our view |
|---|---|---|---|
| Excellent / Ideal | Crisp, even sparkle from across a room | Baseline | Worth paying for |
| Very Good | Very close; most people cannot pick it out | Around 5 to 10 per cent less | Good value, buy on the numbers |
| Good | Softer, a bit sleepier in low light | Around 15 to 20 per cent less | Only if the proportions are borderline Very Good |
| Fair / Poor | Visibly dull or glassy | 30 per cent or more less | Skip it |
| "Super ideal" / hearts and arrows | Marginally tighter light pattern than Excellent | 10 to 25 per cent premium over Excellent | Not worth it for most buyers |
Those percentages move with the market and with the stone, so treat them as a shape rather than a quote. The pattern holds though: the gap between Excellent and Very Good is small in the stone and small in the price, and the gap between Good and Fair is large in both.
Where we would spend, and where we would not
Cut is the one grade we tell people not to compromise on, because it is the only one you can see from across a room. Colour and clarity are inspection-level differences. Cut is not.
That said, super-ideal and hearts-and-arrows stones are where the honest advice gets less flattering to our margins. The optical symmetry is genuinely tighter, and under a scope you can see it. In a ring, on a hand, in normal light, almost nobody can. If you have the budget and you want it, it is a real thing and not a scam. If you are choosing between a super-ideal 0.90 ct and a plain Excellent 1.05 ct at the same price, take the bigger stone.
A useful order of priorities on a fixed budget:
- Cut grade Excellent or Very Good with proportions in range.
- Carat weight that suits the hand and the setting.
- Colour, dropping as far as the metal allows (G to H in white metals, I to J in yellow or rose).
- Clarity, eye-clean is enough. There is a full post on that in the blog.
What to do next
Pick the setting first, because it changes what the stone needs to do. A solitaire with a thin band puts the diamond under a spotlight and rewards a top cut grade; a halo or a heavily detailed setting hides a lot. Browse the engagement ring settings, then filter stones by cut grade and check the proportions against the ranges above.
When you have a setting and a stone you like, put them together in the complete ring builder to choose metal, size and engraving, and rotate the finished ring before you order. If two stones are close on paper, compare them side by side in 3D. The one that looks better on screen is usually the one that looks better in the hand.