How to Read the Cut Grade and Proportions Section of an IGI Diamond Certificate

The cut grade and proportions section of an IGI diamond certificate tells you how well the stone returns light to your eye. For a round brilliant, the numbers to focus on are the overall cut grade (Excellent through Poor), table percentage, total depth percentage, crown angle, and pavilion angle. Together, these five data points predict brightness, fire, and face-up size more reliably than any photograph.

Most buyers scan those numbers quickly and move on. That’s a mistake worth correcting, because two diamonds with identical color and clarity grades can look completely different depending on how they were cut. The proportions section is where that difference lives.

What does the overall cut grade on an IGI certificate actually measure?

The cut grade is not about shape. It reflects how well the diamond’s proportions, polish, and symmetry work together to move light. IGI grades round brilliant diamonds on a five-point scale: Excellent, Very Good, Good, Fair, and Poor. When a round brilliant earns Excellent in cut, polish, and symmetry simultaneously, it is sometimes described as Triple Excellent, a standard that serious buyers treat as a meaningful floor.

For fancy shapes, oval, cushion, pear, and others, IGI does not issue an overall cut grade. Polish and symmetry are still graded separately, so those two fields carry more weight on a fancy-shape report.

One thing worth noting: a Very Good cut grade is not a failure. Many Very Good stones outperform Excellent-graded stones visually, because the grade is an average across many factors. The proportions data below the grade is what lets you check the math yourself.

How do table percentage and depth percentage affect light return?

The table is the large flat facet on top of the diamond. IGI expresses its size as a percentage of the stone’s average diameter. For a round brilliant, a table between 54% and 58% tends to produce the best balance of brightness and fire. Push the table above 62% and the stone can look glassy; drop it below 52% and fire often increases but brilliance suffers.

Total depth percentage compares the stone’s height to its average diameter. The sweet spot for a round brilliant is roughly 61% to 62.5%. A stone outside that range, either too deep or too shallow, tends to leak light through the pavilion or the sides rather than returning it upward through the table. On the certificate, this number appears as a single figure, but it is the sum of crown height, girdle thickness, and pavilion depth, all measured as percentages of diameter.

These two numbers alone will not tell you everything, but they filter out obvious problems fast. If the table is 64% and the depth is 68%, you can move on without needing to see the stone in person.

Why do crown angle and pavilion angle matter more than most buyers realize?

Crown angle is the tilt of the upper facets above the girdle, measured in degrees. It controls how white light disperses into spectral color (fire). A crown angle near 34 to 35 degrees, paired with the right pavilion angle, produces the best mix of brightness and color dispersion in a round brilliant.

Pavilion angle is the tilt of the lower facets below the girdle. This is the most sensitive number on the proportions section. When the pavilion angle sits between 40.6 and 41.0 degrees, light that enters the stone reflects off the lower facets and travels back up through the table. Steepen the pavilion beyond 41.5 degrees and the stone goes dark in the center. Flatten it below 40.4 degrees and you get a “fisheye,” a washed-out reflection of the girdle visible through the table.

Crown and pavilion angle interact. A slightly higher crown angle (around 35 degrees) can compensate for a pavilion at the edge of the ideal range, which is why reading both together matters more than checking either one in isolation.

How do you use the proportions diagram to evaluate a specific stone?

The proportions diagram on an IGI report is a side-profile drawing of the stone with its key measurements annotated. Think of it as a blueprint. It shows table percentage, depth percentage, crown angle, crown height, pavilion angle, pavilion depth, girdle thickness, and culet size in one place.

Start with pavilion angle, since it has the greatest influence on light return. Confirm it falls between 40.6 and 41.0 degrees. Then check the table (54% to 58%) and total depth (61% to 62.5%). Crown angle should sit near 34 to 35 degrees. Finally, look at the girdle: thin to medium is the target. A very thick girdle adds weight without adding visible size, and an extremely thin girdle raises durability concerns.

Culet is the tiny facet at the bottom of the pavilion. Modern well-cut stones list it as None or Very Small. A large culet was common in antique cuts and creates a visible circle when you look down through the table, which is fine if you are buying an old mine cut or rose cut intentionally. For a contemporary round brilliant, None is standard.

Ouros Jewels includes IGI certification on its lab-grown diamond studs and loose stones, so every proportions diagram is available to review before purchase. The IGI Certified Round Lab Grown Diamond Stud Earrings page, for example, provides the full certificate details so you can apply exactly this checklist.

Cut Grade Is What You Are Actually Paying For

Every other quality factor, color, clarity, carat, describes what the diamond is. Cut describes what it does. A 1.00-carat round brilliant cut to Excellent proportions with a table near 56% and a pavilion angle of 40.8 degrees will typically outperform a 1.50-carat stone cut to Poor in every visible measure. The larger stone costs more and looks worse.

When you open an IGI certificate, go to the proportions section before you look at the price. Check pavilion angle first, then table percentage and depth, then crown angle. If those four numbers land in the ranges above, the cut grade printed at the top of the report will make sense. If the proportions are off, the grade alone is not enough to save the stone. For shoppers who want to skip the comparison work, the IGI Certified Radiant Cut Lab Grown Diamond Stud at Ouros Jewels comes with full certificate documentation, so the proportions are there to verify from the start.

How does each grade on the scale change what you see?

Ideal sits at the top of IGI’s scale, above Excellent, and it is specific to IGI. A stone earning Ideal has proportions that produce the maximum measurable light return: tight table percentages typically in the 54 to 57 percent range, depth around 59 to 62.6 percent, and crown angles near 34 to 35 degrees. The visual result is a stone that looks bright and evenly patterned from every angle.

Excellent is one step below Ideal. The proportions remain very tight, and the difference in face-up appearance is almost impossible to spot with the naked eye in normal lighting. For most buyers, Excellent represents the practical ceiling: it is widely available, and the price gap between Excellent and Ideal tends to be modest.

Very Good reflects most of the light entering the stone, but some facets may be slightly misaligned. The difference between Very Good and Excellent is often visible under direct lighting, with slightly less evenness in the sparkle pattern.

Good allows some light to escape through the base as leakage. The stone can still look appealing, but it will appear noticeably less lively than an Excellent-cut stone of the same color and clarity.

Fair and Poor represent proportions that cause significant light leakage. A Fair stone tends to look flat or glassy in most lighting; a Poor stone will show prominent dark areas face-up. These grades rarely appear in fine jewelry retail, but they do exist in the wholesale market.

Why does Triple Excellent matter specifically for lab grown diamonds?

Lab grown diamonds are physically and optically identical to mined diamonds, so the same grading criteria apply. But there is a practical reason Triple Excellent carries particular weight in the lab grown category.

Because lab grown production has scaled significantly over the past several years, the market now contains a wide range of cutting quality. A grower can produce rough in a controlled environment, but the cutting is still done by hand, and that is still where precision varies. IGI is the most widely accepted certification body for lab grown diamonds, and it applies the full grading scale to every stone it processes. When you see Triple Excellent on an IGI lab grown certificate, you know the cutting team made no compromises on any of the three craftsmanship grades.

The grade spread also matters for price comparison. Two IGI-certified lab grown round brilliants at the same carat weight, color, and clarity can carry noticeably different prices depending on cut quality. A stone graded Excellent/Very Good/Very Good on cut, polish, and symmetry will typically cost less than a 3EX stone, and the optical difference, particularly in fire and scintillation, is often visible in normal indoor lighting.

Does Triple Excellent apply to all diamond shapes?

No, and this is where buyers sometimes get confused. The Triple Excellent designation applies exclusively to round brilliant cut diamonds. Fancy shapes such as ovals, cushions, emeralds, and pears do not receive an overall cut grade from IGI. They are assessed on polish and symmetry only, which means the best a fancy shape can achieve is a “Double Excellent” in those two categories.

This is not a flaw in the grading system. Fancy shapes have no standardized ideal proportions the way round brilliants do, so assigning a single cut grade would be misleading. For fancy shapes, buyers should focus on the individual polish and symmetry grades, along with the stone’s specific measurements, to evaluate light performance.

If you are shopping for a round brilliant and the certificate does not list all three grades, or lists them without specifying the level, ask the retailer to confirm each one individually before purchasing.

How does IGI actually assign a color grade?

The process is more controlled than most buyers realize. IGI gemologists place the diamond upside down and view it through the pavilion, a position chosen specifically to neutralize the stone’s brilliance so that body color can be read without interference from light return. The stone is then compared against a set of calibrated master color stones under standardized lighting. Multiple graders assess the same diamond independently, with no collaboration between them, and a grade is confirmed only when there are sufficient agreeing opinions.

That protocol matters for one practical reason: the grade is not influenced by how the diamond looks face-up in a ring. The upside-down viewing method isolates body color, which is why a stone graded G or H by IGI will often appear whiter than that letter suggests once it is set and lit from above.

What does each color group on the D to Z scale look like?

The scale breaks into four meaningful bands, and each one behaves differently in jewelry.

D, E, F (Colorless) sit at the top of the scale. These stones carry no detectable tint, even when viewed under controlled laboratory conditions by a trained gemologist. They are the rarest and most expensive grades, and they pair naturally with platinum or white gold settings that amplify their icy appearance.

G, H, I, J (Near-Colorless) make up the most commercially active part of the scale. Face-up in a ring, a G or H stone is essentially indistinguishable from a D or E to the naked eye. An I or J may show a faint warmth when compared directly to a colorless stone under strong lighting, but that warmth becomes far less visible in yellow or rose gold settings, where the metal’s own tone absorbs it.

K, L, M (Faint Color) carry a warm tint that is visible without any direct comparison, though some buyers actively seek this look for vintage or antique-inspired designs. Yellow gold settings work well here, since the metal blends with rather than contrasts against the stone’s tone.

N through Z (Very Light to Light Color) show progressively stronger yellow or brown. Diamonds in this range are rarely used as center stones in fine jewelry, and IGI notes that controlled lab growth rarely produces strong yellows or browns in this range anyway. When it does, those tones are usually engineered intentionally and graded as fancy colors under a separate system entirely.

Which color grade offers the best value for a lab grown diamond buyer?

For most buyers choosing a white metal setting such as platinum or 14k white gold, G or H is the practical sweet spot. Both grades appear colorless face-up, work in any metal color, and carry a meaningful price advantage over D through F. If the setting is yellow or rose gold, an I or J grade becomes viable, because the warm metal tone neutralizes whatever faint tint the stone carries. Going below J in any setting tends to introduce visible warmth that most buyers find distracting in a center stone.

One variable worth accounting for is carat weight. Larger diamonds show body color more easily because there is simply more mass for tint to accumulate in. A 0.75-carat H-color stone will face-up whiter than a 2-carat H-color stone in the same setting. If you are buying above 1.5 carats, moving up to G or even F becomes a more defensible choice.

Accent stones and side diamonds follow a different rule. They typically sit one to three grades below the center stone without creating a visible mismatch, so there is no reason to pay for colorless grades in small side settings.

What does the clarity plot actually show?

The plot is a bird’s-eye outline of the diamond’s shape, usually shown from two angles: the crown (looking down through the table) and the pavilion (looking up from the bottom). Graders draw each clarity characteristic to scale in its approximate location as seen under 10x magnification, so the diagram reflects the stone’s internal geography as faithfully as a small printed image allows.

The color coding is the first thing to learn. Red symbols represent inclusions, meaning characteristics enclosed within the stone. Green symbols represent blemishes, meaning surface features. On most IGI full reports, a legend alongside the plot identifies the specific symbol shapes: a small dot for a pinpoint (a tiny crystal too small to resolve individually), a circle or irregular shape for a crystal inclusion, a short line for a feather (a small internal fracture), and a hazy outlined region for a cloud (a cluster of pinpoints so dense they read as a single area).

Some IGI reports, particularly those issued on condensed formats, omit a full symbol legend and note only that red means internal and green means external. If your report is one of those, cross-reference the Comments field on the certificate, which often names the most significant characteristics by type.

How does inclusion position change what you actually see on your finger?

This is where the plot earns its keep. The clarity grade reflects how visible an inclusion is under a 10x loupe, but it cannot tell you whether it will catch your eye across a dinner table. Position is the variable the grade simply cannot capture on its own.

An inclusion sitting directly under the table facet (the large flat top of the diamond) is in the worst possible location. It sits in the center of the face-up view, with no facet geometry to scatter or hide it. A dark crystal there on an SI1 can be visible to the naked eye. By contrast, the same type of inclusion tucked near the girdle edge (the narrow band around the diamond’s perimeter) is often hidden by a prong in a four-claw setting, or simply too far from center to catch attention.

Step-cut shapes such as emerald and Asscher cuts deserve extra attention here. Their long parallel facets act like mirrors and put the interior of the stone on full display, so an inclusion that disappears inside a round brilliant’s light scatter can be clearly visible in an emerald cut at the same clarity grade. For step cuts, VS2 is generally the minimum grade worth considering, and the plot still warrants a close look even then.

Brilliant cuts (round, oval, cushion, radiant) are more forgiving. Their facet pattern scatters light in ways that mask a lot. An SI1 round with inclusions clustered near the girdle can be genuinely eye-clean, while an SI1 with a single dark crystal under the table may not be. The plot is what tells you which one you have.

What does the Comments field add to the clarity picture?

The Comments section is the most skipped part of any diamond certificate and occasionally the most important. It records clarity characteristics that were not drawn on the plot, either because they were too faint or too widespread to map usefully.

The phrase that should stop you cold is: “clarity grade is based on clouds not shown.” This means the characteristic setting the grade is a diffuse haze spread through the stone that the grader could not usefully represent as a single symbol. A stone with that note can appear flat or milky in daylight even though its printed grade looks respectable. The plot will look almost empty, which is misleading rather than reassuring.

Treatments are also disclosed here. For lab-grown diamonds, an HPHT post-growth treatment used to improve color will appear in Comments. Full IGI grading reports include the Comments field alongside the clarity plot, so both are present and readable before you buy, rather than buried in a condensed card format that omits them.

What is IGI actually measuring in the fluorescence section?

Fluorescence is a physical property of the diamond crystal itself. When certain structural elements in the stone absorb ultraviolet energy, they release it almost immediately as visible light. That release is what you see as the glow. IGI tests each stone by exposing it to a standardized UV source and recording both the intensity and the color of any emission.

The color is worth noting. Blue is by far the most common, appearing in over 95% of fluorescent diamonds. Occasionally a stone will emit yellow, white, or orange light instead, and IGI records whichever color is present. The intensity scale runs from None (no reaction) through Faint, Medium, Strong, and Very Strong.

One number worth keeping in mind: roughly 25 to 35% of natural diamonds show some fluorescence, while lab-grown diamonds show it far less often, with IGI’s own data suggesting around 10% or fewer of lab-grown stones submitted for grading exhibit any fluorescence at all. So if your IGI report for a lab-grown diamond says None, that is entirely normal and expected.

What does each fluorescence grade mean in plain terms?

None is the baseline. The stone shows no reaction to UV light, and this grade carries no price adjustment in either direction. It is also the most common grade across all diamonds.

Faint means a very mild glow is detectable under a UV lamp, but it is essentially invisible under any normal lighting condition, including direct sunlight. The visible effects of Faint fluorescence are perceptible only to a trained gemologist using a dedicated UV source. In practice, Faint has no meaningful impact on how the diamond looks when worn.

Medium is where things get more interesting, and where your color grade starts to matter. For a colorless stone (D, E, or F), Medium fluorescence can occasionally create a very slight reduction in transparency, though most observers would never notice it. For a near-colorless stone in the G-to-J range, Medium blue fluorescence can work in your favor: because blue and yellow are complementary colors, the blue glow under daylight UV can partially cancel out any faint yellow tint, making the stone appear up to a full grade whiter to the naked eye.

Strong and Very Strong are the grades that warrant closer attention. Strong fluorescence can sometimes cause a hazy or oily appearance in colorless diamonds, though research consistently shows this happens in only a minority of stones, not as a rule. Very Strong carries a slightly higher risk of the same effect. If you are looking at a D-to-F stone with either of these grades, it is worth viewing the specific diamond in person or requesting video footage in different lighting before buying.

Does fluorescence change what you should pay?

Yes, and the relationship is not symmetric. The market applies a discount to fluorescent diamonds in colorless grades and a smaller or neutral adjustment in near-colorless and tinted grades.

For D-to-F diamonds, Strong fluorescence typically reduces price by roughly 10 to 25% compared to an otherwise identical stone graded None. Faint fluorescence in the same color range tends to carry a smaller discount of around 2 to 5%, often with no meaningful difference in appearance. For a D-color diamond with Medium fluorescence, market data suggests prices run approximately 18% lower than the non-fluorescent equivalent.

In the G-to-K range, the picture changes. Medium blue fluorescence at these color grades is often priced at parity with None, and in some near-colorless and tinted stones it may carry a slight premium because it visibly improves the face-up color in natural light. A J-color diamond with Medium blue fluorescence can face up like an H-color stone in daylight, which is a real value advantage if you understand it.

The discount on colorless fluorescent stones is largely driven by market perception rather than a proven optical defect. Buyers who understand the science can sometimes find well-cut, beautiful D-or-E stones at meaningfully lower prices simply because fluorescence is listed on the report.

Frequently Asked Questions

Does IGI grade the cut of fancy-shaped diamonds the same way it grades round brilliants?

No. IGI issues an overall cut grade only for round brilliant diamonds. For fancy shapes such as oval, cushion, pear, and princess, the certificate reports polish and symmetry grades separately but omits a combined cut grade. For those shapes, pay closer attention to the proportions diagram and the symmetry grade to assess light performance.

What is a good total depth percentage for a round brilliant on an IGI certificate?

For a round brilliant, a total depth between 61% and 62.5% is generally considered the optimal range for light return. Stones deeper than 64% tend to look smaller face-up and can show a dark center. Stones shallower than 59% often appear washed out because light exits through the pavilion sides rather than returning upward.

Can two diamonds with the same IGI cut grade look noticeably different in person?

Yes, and this happens often. The overall cut grade is an average across many proportional factors. Two Excellent-graded stones can have slightly different pavilion angles or table percentages that sit at opposite ends of the acceptable range, producing different levels of fire and brightness. Checking the individual proportions numbers, not just the headline grade, is how you distinguish them.

Is a Very Good IGI cut grade worth buying, or should I always target Excellent?

A Very Good cut grade is worth serious consideration. Many Very Good stones have proportions that fall within the Excellent range for the most critical measurements, particularly pavilion angle, and simply score slightly outside the threshold on a less influential factor. Review the proportions data directly rather than filtering by grade alone, and you will often find strong performers at a lower price point.

What does the girdle thickness listed on an IGI certificate tell me about durability?

Girdle thickness affects both durability and face-up size. A very thin girdle increases the risk of chipping at the stone’s widest point, particularly during setting. A very thick girdle adds carat weight that is hidden in the setting rather than contributing to visible size. Thin to medium is the standard recommendation for round brilliants, and IGI grades girdle thickness on a scale from Extremely Thin to Extremely Thick.