Diamond Cut Grades Explained: A UK Buyer's Guide

Diamond cut grade is a measure of how well a diamond has been cut and polished — not its shape. It grades how efficiently a diamond returns light to the eye, and it is the single biggest driver of sparkle. On a GIA report, cut is graded as Excellent, Very Good, Good, Fair, or Poor, and it applies to round brilliant diamonds. For most UK engagement rings, an Excellent or Very Good cut delivers the strongest visual performance for the money.

Two diamonds with identical carat weight, colour and clarity can look dramatically different in real light. The one that sparkles more is almost always the better-cut stone. That is why, at Diamond Hub, we treat cut as the C that buyers should compromise on last.

What Diamond Cut Grade Actually Means

Cut grade evaluates the craftsmanship of the diamond — the proportions, symmetry and polish that determine how light travels through it. It is judged on three optical effects:

• Brightness — the white light reflected back from the diamond.
• Fire — the flashes of coloured light (spectral dispersion) that a well-cut diamond produces.
• Scintillation — the pattern of light and dark, and the sparkle when the diamond moves.

A well-cut diamond captures light through the crown, bounces it internally off the pavilion facets, and sends it back out through the top. A poorly cut diamond leaks light out of the sides or the bottom, so it looks dull, glassy or lifeless even if the other Cs look excellent on paper.

Cut Grade vs Diamond Shape

This is the single most common point of confusion for UK buyers, so it is worth being explicit:

• Shape = the outline of the diamond: round, oval, princess, emerald, cushion, pear, marquise, radiant, Asscher, heart.
• Cut grade = how well that shape has been cut.

Only round brilliant diamonds receive an official GIA cut grade. Fancy shapes (oval, pear, emerald, etc.) are assessed on polish and symmetry, and buyers rely on proportions, ratios and visual review to judge cut quality. If you are choosing a round diamond, cut grade is a headline number. If you are choosing a fancy shape, the equivalent quality lives in the proportions section of the certificate and in seeing the stone in person.

The Cut Grade Scale

GIA (the Gemological Institute of America) uses five cut grades. IGI (International Gemological Institute) and most other credible labs use a comparable five-tier scale.

Excellent (EX)

Excellent-cut diamonds return the maximum amount of light. Proportions, symmetry and polish are all in the top tier. This is the grade to aim for if you want the brightest, liveliest stone. Around the top 3% of diamonds fall into this category.

A diamond graded Triple Excellent (sometimes shortened to 3EX) has Excellent cut, Excellent polish and Excellent symmetry. This is generally considered the benchmark for optical performance in a round brilliant.

Very Good (VG)

Very Good diamonds reflect nearly as much light as Excellent, at a lower price point. To the untrained eye, and often even to a trained one, a well-chosen Very Good diamond can be visually indistinguishable from Excellent. This grade is a strong value option for buyers optimising budget without compromising sparkle noticeably.

Good

Good diamonds show reasonable brilliance but not the full fire of the higher grades. Light performance is compromised in ways that a buyer will start to see, particularly when comparing side by side with a better-cut stone.

Fair

Fair-cut diamonds are noticeably less brilliant. Proportions have been sacrificed to preserve carat weight from the rough diamond. Not recommended for engagement rings where sparkle matters.

Poor

Poor-cut diamonds look dull. Light leaks out of the pavilion or the sides, and the stone lacks life. Almost always the wrong choice for an engagement ring at any price.

Why Cut Matters More Than the Other Cs

Cut grade is the C that is fully under human control. Colour and clarity are largely determined by nature. Carat is the size of the rough. But cut is a decision — the diamond cutter chooses whether to prioritise weight retention or optical performance. That decision is what buyers are paying for.

The practical consequence:

• A well-cut, lower-colour diamond will usually look brighter and whiter than a poorly cut, higher-colour diamond.
• A well-cut SI1 clarity diamond will usually look cleaner and livelier than a poorly cut VVS.
• A slightly smaller but Excellent-cut diamond will often look larger to the eye than a heavier stone with a deep or shallow cut, because a well-proportioned diamond spreads correctly across the finger.

This is why our advice at Diamond Hub is consistent: pick cut first, then balance the other three Cs to your budget.

How Cut Grade Is Judged

For round brilliants, GIA grades cut based on seven components:

1. Brightness — white light reflection.
2. Fire — dispersion of light into spectral colours.
3. Scintillation — sparkle and light–dark pattern.
4. Weight ratio — carat weight relative to diameter.
5. Durability — girdle thickness and culet size.
6. Polish — surface finish quality.
7. Symmetry — precision and alignment of the facets.

The first three are the optical outcomes. The last four are the physical measurements and finish that produce those outcomes. All are visible on a full GIA grading report.

Key Proportions to Look For

For a round brilliant, the following proportions tend to correlate with Excellent cut performance:

• Table percentage: 54–58%
• Depth percentage: 60–62.5%
• Crown angle: 34–35°
• Pavilion angle: 40.6–41°
• Girdle: thin to slightly thick, faceted
• Culet: none or very small

You do not need to memorise these to buy a beautiful diamond. But if you are comparing two similarly priced Very Good stones and one falls within these ranges while the other doesn't, the one that does will almost always look better.

What Cut Grade to Choose for a UK Engagement Ring

For a round brilliant engagement ring, we recommend the following framework:

• Excellent (ideally Triple Excellent): the default choice for anyone prioritising sparkle. Best light performance, best resale-relevant quality, best presentation under indoor and jewellery lighting.
• Very Good: the smart-value choice. Look for Very Good cut with Excellent polish and Excellent symmetry, and check proportions fall inside the ideal range. In the hand, most buyers cannot distinguish this from Excellent.
• Good, Fair, Poor: avoid for a serious engagement ring purchase.

For fancy shapes, since there is no headline cut grade, focus on:

• Polish and Symmetry: aim for Very Good or Excellent on both.
• Length-to-width ratio: appropriate for the shape (for example, 1.35–1.50 for a classic oval; 1.30–1.60 for pear).
• Depth and table: within accepted ranges for the shape.
• In-person or high-quality video review: essential for fancy shapes, especially for detecting the "bowtie" effect in ovals, pears and marquises.

How Cut Grade Appears on a Diamond Certificate

On a GIA or IGI certificate, cut grade appears alongside polish and symmetry, usually near the top of the "Grading Results" section. You will see three lines that matter:

• Cut Grade: Excellent / Very Good / Good / Fair / Poor
• Polish: Excellent / Very Good / Good / Fair / Poor
• Symmetry: Excellent / Very Good / Good / Fair / Poor

If all three read Excellent, the diamond is "Triple Excellent." If cut is Excellent and polish or symmetry drops to Very Good, the diamond is often described as "GIA EX, VG polish" — still an excellent choice, at a small saving. Our full guide on how to read a diamond certificate walks through the rest of the report in detail.

How Diamond Hub Approaches Cut

Every certified diamond we source for bespoke engagement rings is reviewed with cut grade as the anchor. In practice, that means:

• For round brilliants, we default to Excellent or Triple Excellent cut and only consider Very Good when a client has a specific reason (usually budget optimisation for a larger carat).
• For fancy shapes, we hand-pick stones on proportions, polish, symmetry and eye-visible performance, not just paper grades.
• We show clients diamonds in person or on high-quality video so cut can be judged the way it will actually be worn — under real light, on real skin, from real angles.

If you're commissioning a bespoke piece, cut is where we start the diamond conversation, not where we finish it. Explore the bespoke engagement ring service or browse our engagement ring collection to see well-cut stones in a range of settings.

What to Consider Before Buying

Before committing to a diamond, run through the following:

• Is the diamond certified by GIA or IGI? These are the two most respected labs for cut grading. Certification protects your investment and gives you a defensible cut assessment.
• What is the cut grade — and the polish and symmetry? Look at all three, not just the headline.
• Have you seen the diamond in real light or on high-quality video? Photographs are not enough for cut assessment.
• Are you prioritising cut over the other Cs? If you are choosing between spending more on carat or spending more on cut, cut almost always wins visually.
• Are you comparing like with like? Two diamonds at the same price with the same carat can vary hugely in cut. Ask for cut proportions on the certificate before comparing.

FAQ

What is the best cut grade for a diamond engagement ring? Excellent cut is the best grade. It delivers the strongest brightness, fire and scintillation. For value, Very Good cut with Excellent polish and Excellent symmetry is a strong alternative that most buyers cannot visually distinguish from Excellent.

Is a Triple Excellent diamond worth the extra cost? Usually yes, if you are buying a round brilliant. Triple Excellent means Excellent cut, polish and symmetry — the highest tier of optical performance. Over a few thousand pounds and above, the price uplift for Triple Excellent is generally modest relative to the visual improvement.

Is diamond cut the same as diamond shape? No. Shape is the outline (round, oval, princess, emerald, pear, etc.). Cut grade is the quality of the cutting workmanship. Only round brilliant diamonds receive an official GIA cut grade; fancy shapes are assessed via polish, symmetry and proportions.

Does cut grade matter more than colour or clarity? Yes, for how the diamond looks. A well-cut lower-colour diamond will usually appear whiter and livelier than a poorly cut higher-colour diamond. Cut is the C that most directly controls the sparkle you actually see.

Do fancy shapes like oval or emerald have a cut grade? Not an overall cut grade, no. GIA only issues an overall cut grade for round brilliants. For fancy shapes, look at polish, symmetry and proportion ratios on the certificate, and always review the stone visually before buying.

Can I see the diamond's cut on the certificate? Yes. On a GIA report, cut grade appears in the Grading Results section alongside polish and symmetry, with full proportions displayed in a diagram lower on the report. IGI certificates present similar information.

Conclusion

Cut grade is the difference between a diamond that impresses and one that disappears in daylight. It is the C that controls sparkle, and it is the C worth prioritising when you build an engagement ring. Choose Excellent (or a well-selected Very Good) for a round brilliant, and rely on proportions and expert review for fancy shapes.

If you would like help selecting a certified diamond with the right cut for your ring, our team can talk you through the trade-offs in a private consultation. Explore the bespoke engagement ring service or browse engagement rings to see how cut translates into presence on the finger.