Diamond Cut Grade Explained

Of the four Cs, cut is the one most buyers overlook and the one that matters most for how the diamond actually looks. Color and clarity are about what the diamond is; cut is about how it behaves. A perfectly colorless, flawless diamond with a poor cut looks dull. A near-colorless, slightly included diamond with an Excellent cut looks electric. This guide explains what a diamond cut grade actually measures, how the GIA and AGS scales work, which grade to buy, and why spending on cut gives you more sparkle per dollar than any other upgrade.

At HOLYCOME, we see buyers negotiate hard on color and clarity while accepting a mediocre cut, then wonder why the ring does not look as bright as the one in the advertisement. The difference is almost always the cut. Understanding cut grades is the single highest-leverage piece of diamond knowledge you can acquire. It is also the most technical of the four Cs, because it is the only one that is measured by geometry rather than by comparison to a master set.

This guide is written for buyers and wholesalers who want to understand what the cut grade on a report actually means. We cover the GIA and AGS scales, the numbers behind the grade, the price differences between grades, and the common mistakes that lead people to overpay or under-buy. By the end, you should be able to look at a cut grade and know whether the stone in front of you is a good cut, a great cut, or a poor one. The cut grade is not a marketing label; it is a measurement of optical performance, and once you understand it, you will never buy a poorly cut diamond again.

Why cut is the four C that creates brilliance

A diamond's sparkle is not the stone itself shining; it is light entering through the top, bouncing off the inner facets, and returning to your eye. When the angles are right, almost all the light that enters comes back out the top, and the stone flashes. When the angles are wrong, light leaks out the bottom or the sides, and the stone looks dark or glassy. Cut grade is the lab's measurement of whether those angles are right.

This is why cut is the only four C that the cutter controls. Color, clarity and carat weight are gifts of nature; the cutter decides how to shape the rough. A skilled cutter can choose to preserve weight by making a deep, dark stone, or to sacrifice weight for brilliance by making a well-proportioned one. The cut grade tells you which choice the cutter made. A high cut grade means the cutter prioritized sparkle over weight recovery.

The visual effect is dramatic. Stand two one-carat diamonds side by side, one with an Excellent cut and one with a Fair cut, under normal room light. The Excellent one will catch the eye across the room; the Fair one will look like a piece of glass. No amount of color or clarity upgrade compensates for a poor cut. This is the reason gemologists put cut first when they rank the four Cs by visual impact.

What cut grade actually measures

For a round brilliant, the GIA cut grade is a composite score built from several measurements. The lab measures the table percentage, the depth percentage, the crown angle, the pavilion angle, the star facet length, the lower-half length, the girdle thickness, the polish and the symmetry. Each of these affects how light travels through the stone. The lab combines them into a single letter grade: Excellent, Very Good, Good, Fair or Poor.

The AGS uses a numeric scale from 0 to 10 instead. AGS 0 is ideal (the top), AGS 1 is excellent, AGS 2 is very good, and so on down to AGS 10. The AGS scale is finer-grained and is favored by cut specialists, because it distinguishes between truly ideal cuts and merely excellent ones. For most buyers, a GIA Excellent and an AGS 0-1 are equivalent top-tier stones.

For fancy shapes (princess, oval, cushion, pear, emerald), GIA does not assign a single cut grade. Instead it lists polish and symmetry grades, and the buyer judges the proportions by eye. This is why fancy-shape buying requires more personal inspection: there is no single letter to rely on. The proportions and the length-to-width ratio are a matter of taste, and you should look at the stone in person.

The GIA cut grade scale

The GIA scale runs from Excellent at the top to Poor at the bottom. Excellent is the top tier, awarded to stones whose proportions place almost all light return in the ideal range. About a quarter of round brilliants graded by GIA earn Excellent. Very Good is the next tier; light return is close to ideal, and the stone looks bright to the naked eye. Very Good stones cost less than Excellent and are a common value pick.

Good is the middle of the road. A Good-cut stone is acceptable but visibly less brilliant than a Very Good or Excellent one, especially in larger sizes. Fair and Poor cuts are where light leaks out significantly; these stones look dull and should be avoided for engagement rings, even at a discount. A Fair-cut diamond may be cheap per carat, but it is cheap for a reason.

When you are comparing prices, remember that the cut grade compounds with size. On a half-carat stone, the difference between Excellent and Very Good is subtle. On a two-carat stone, it is obvious. The larger the diamond, the more the cut grade matters, because there is more area for light to leak through. Budget-conscious buyers on small stones can accept Very Good; buyers on large stones should insist on Excellent.

The AGS cut grade scale, 0 to 10

The American Gem Society scale runs from 0 (ideal) to 10. An AGS 0 is the strictest cut grade in the industry, and very few stones earn it. AGS 1 is excellent, AGS 2 is very good, and so on. The AGS system is popular among buyers who care about optical performance, because it breaks the top tier into finer steps. A stone graded AGS 0 is not just excellent; it is in the top few percent of cuts.

The AGS also publishes a separate light-performance score for some stones, measuring brightness, fire and scintillation with an instrument. This is a more objective measure than the human-judged GIA grade, but it is less commonly used. For most buyers, the AGS numeric grade is enough: aim for 0, 1 or 2.

AGS reports are less common than GIA, especially in the wholesale market. A stone with an AGS 0 grade is usually a purpose-cut ideal diamond, often priced at a premium. If you see one at a competitive price, it is worth serious consideration, because the cut is genuinely exceptional.

For a wholesaler, AGS stones are a niche product. They sell to cut enthusiasts and to buyers who specifically want the strictest cut grade. They are not the volume product. The volume is in GIA Excellent and Very Good stones, which cover most of the market. At HOLYCOME, we stock both: GIA Excellent for the mainstream, and the occasional AGS 0 for the specialist buyer.

Ideal, Excellent and Very Good explained

The word 'ideal' is used in two ways. Historically, the American Ideal cut (also called the Tolkowsky cut) was a set of proportions derived by Marcel Tolkowsky in 1919. Modern 'ideal' stones are cut to slightly different proportions, often based on newer optical research. When a seller says 'ideal cut', they usually mean a top-tier stone, but the exact meaning depends on whether the stone carries an AGS 0 or a GIA Excellent.

'Excellent' on a GIA report is the top of the GIA scale. It is a broad grade that includes both true ideal cuts and stones that are simply very good. A GIA Excellent is not automatically an AGS 0; the GIA grade is more forgiving. If you want the strictest top grade, look for an AGS 0. If you want a reliable, widely recognized top grade, GIA Excellent is fine.

'Very Good' is the value sweet spot. A Very Good cut stone looks nearly as bright as an Excellent one in normal light, and it costs 10 to 20 percent less. For a buyer on a budget, upgrading from Good to Very Good is worth more than upgrading from SI1 to VS2. The visual difference between Very Good and Excellent is subtle; the price difference is not.

For a wholesaler, the cut grade is also a marketing point. A stone advertised as Excellent-cut sells faster and at a higher margin than a Very Good stone, even when the visual difference is small. The Excellent label carries weight with retail buyers who have read the buying guides. This is why factories optimize for Excellent grades when they can, and why the Excellent premium is embedded in the market.

When you are choosing between two Excellent-cut stones at the same price, look at the actual proportions. Two Excellents can be tuned differently: one may have more brilliance, the other more fire. If you have a preference, ask the dealer which stone shows more white flash versus colored flash. It is a subtle difference, but it is the difference between two stones that look technically the same.

Depth and table percentages

The two numbers on the report that tell you the most about cut quality are the table percentage and the depth percentage. The table is the size of the large flat top facet, expressed as a percentage of the diamond's diameter. For an ideal round, the table is typically 53 to 58 percent. A table over 65 percent means the top facet is too large and the stone will look less lively.

The depth percentage is the height of the diamond divided by its diameter. Ideal rounds sit around 59 to 62 percent. A depth over 65 percent means the stone is too deep, and light leaks out the bottom. A depth under 57 percent means the stone is too shallow, and light leaks out the sides. Both extremes kill brilliance, and both are reflected in a lower cut grade.

When you are comparing two stones with the same cut grade, the table and depth percentages tell you which one is better within the grade. Two GIA Excellent stones can have slightly different proportions, and the one closer to the ideal range will usually look better. Ask the seller for the proportions sheet; it is printed on every GIA and AGS report.

For a round brilliant, the ideal table is around 55 to 58 percent. Above 62 percent, the top facet looks too large and the stone loses fire. Below 52 percent, the stone starts to look too dense. The ideal depth is 59 to 62 percent. Outside these ranges, the cut grade drops. If you see a stone with a 70 percent table and a cut grade of Excellent, treat the grade with skepticism and look at the stone in person.

Polish and symmetry

Polish measures how smooth the facets are. A poor polish leaves tiny scratches or abrasions on the facet surfaces, which scatter light and reduce brilliance. Symmetry measures how well the facets align; poor symmetry means the facets are slightly off-center, which can make the stone look lopsided or create a 'bow-tie' dark area on fancy shapes.

Both polish and symmetry are graded Excellent, Very Good, Good, Fair or Poor. On a modern round brilliant, you should expect Excellent or Very Good on both. A Good symmetry on an oval or pear can create a visible bow-tie shadow across the middle, which some buyers accept and others hate. Inspect the stone in person before you buy a fancy shape with Good symmetry.

Polish and symmetry are minor compared to the main proportions, but they are cheap to upgrade. A stone with Excellent polish and symmetry looks crisper than one with Good, even when the cut grade is the same. If you are choosing between two stones with identical cut grades, pick the one with Excellent on both.

For a wholesaler, polish and symmetry are also easy selling points. A stone with Excellent on both is an easier sale than one with Good, because the retail buyer has read that Excellent polish and symmetry is better. The cost to the cutter is small, and the marketing benefit is real. At HOLYCOME, our round brilliants all carry Excellent or Very Good polish and symmetry, because we want the stones to perform well in any setting.

When you inspect a stone under magnification, look at the facets. Are they clean and sharp, or do they show tiny abrasions? A well-polished facet reflects light like a mirror; a poorly polished one scatters it. The difference is visible under 10x, and it adds up across the whole stone. This is one of the cheapest upgrades in diamond buying, and there is no reason to accept less than Excellent.

How cut grade affects price

Cut grade has a large effect on price, because the cutter loses weight when making a well-proportioned stone. A rough diamond that yields a 1.0-carat Excellent cut could have yielded a 1.2-carat Good cut. The cutter chooses the Excellent cut knowing they sell less weight but at a higher price per carat. That tradeoff is why Excellent-cut stones cost more per carat than Good-cut ones.

For the buyer, the pricing pattern is familiar. Going from Good to Very Good adds maybe 10 to 15 percent to the price per carat. Going from Very Good to Excellent adds another 10 to 15 percent. Going from Excellent to AGS 0 adds more again. The visual improvement per dollar is highest at the Very Good to Excellent step, and diminishes as you move into the rarefied ideal range.

There is also a size effect. The price jump between cut grades is larger on bigger stones, because a poorly cut two-carat stone is a more obvious disappointment than a poorly cut half-carat. If you are buying a larger stone, the case for Excellent cut is even stronger. If you are buying a small stone under half a carat, a Very Good cut is perfectly fine and saves you money.

When you compare prices, look at the price per carat, not the total price. A 1.0-carat Excellent stone at $6,000 per carat and a 0.9-carat Very Good stone at $5,000 per carat may cost similar totals, but the Excellent stone will look better. The cut grade is embedded in the per-carat price, and comparing per-carat numbers is how you see the cut premium clearly.

This is why the standard budget advice is: buy the best cut you can afford, then fill in color and clarity around it. If your budget is tight, drop to a G color and an SI1 clarity to afford an Excellent cut, rather than buying a D color and VS2 in a Good cut. The Excellent-cut G SI1 will look more beautiful on the finger than the Good-cut D VS2.

This advice holds whether you are buying a natural diamond or a lab-grown one. On a grown diamond, the color and clarity are already high because the material is grown under controlled conditions. The cut is the variable. At HOLYCOME, our grown diamonds are cut to optimized proportions specifically to maximize brilliance, and the cut grade on the report reflects that. If you are comparing two grown stones of the same price, pick the one with the better cut.

Proportion diagrams and ideal cut angles

Behind every cut grade is a set of numbers. The crown angle is the angle between the top facet and the girdle; ideal round brilliants have a crown angle around 34 to 35 degrees. The pavilion angle is the angle of the lower facets; ideal is around 40 to 41 degrees. If the pavilion angle is too shallow, light leaks out the bottom; if it is too steep, light leaks out the sides. The cut grade is the lab's summary of whether these angles land in the sweet spot.

The star facet length and lower-half length control the balance between brilliance (white flashes) and fire (colored flashes). Longer star facets create more fire; shorter ones create more brilliance. There is no universally perfect combination; it is a matter of optical taste. AGS publishes light-performance scores that measure brightness, fire and scintillation separately, which lets buyers pick the optical personality they prefer.

If you are a cut enthusiast, the proportion table on the report is worth studying. Two GIA Excellent stones can have slightly different crown and pavilion angles, and one may look brighter while the other shows more fire. For a normal buyer, the cut grade is enough. For a connoisseur, the numbers on the table are where the real choice lives.

Cut on fancy shapes

Fancy shapes do not get a single GIA cut grade, but they have proportions that matter. An oval should have a length-to-width ratio around 1.3 to 1.5 for a classic look. A princess should be close to square. A cushion should have soft corners and a length-to-width ratio near 1.1 to 1.2. If the proportions are off, the shape looks wrong, regardless of how well the individual facets are cut.

Fancy shapes also show bow-tie: a dark shadow across the middle of an oval or pear. Bow-tie is caused by light leakage through the pointed ends. A slight bow-tie is normal and accepted; a heavy bow-tie makes the stone look dark. The only way to judge bow-tie is to look at the stone in person, because a photo does not capture it. Never buy an oval or pear sight unseen.

A poorly cut stone wastes the rough. The cutter could have produced a brighter stone at the same weight, but chose not to. When you buy a poorly cut diamond, you are paying for weight that looks dull. The cut grade is the lab's way of telling you which stones were cut for beauty and which were cut for weight.

Symmetry on fancy shapes is more important than on rounds. A lopsided oval looks obviously wrong, even if the individual facets are well cut. On the report, look for Excellent symmetry on any fancy shape. Good symmetry is acceptable on a budget stone, but inspect it carefully. For more on the other Cs, start with our guide to the diamond 4Cs, and for the lab that defines the ideal, see AGS certification explained.