What Does the Cut Grade on an IGI Certificate Actually Mean?

The cut grade on an IGI certificate tells you how well a diamond’s facets interact with light, scored on a scale of Ideal, Excellent, Very Good, Good, Fair, and Poor. It is the single grade most directly tied to brilliance: a poorly cut stone loses light through its base, while a well-cut stone sends it back through the crown as sparkle. Of the 4Cs, cut is the only one entirely controlled by the cutter after the rough diamond exists, which is why IGI treats it as the most detailed section of the report.

That detail matters when you are holding an IGI certificate for a lab-grown diamond and trying to figure out what you are actually paying for. Color and clarity affect appearance in subtle ways; cut affects it in ways visible to anyone in ordinary room lighting.

What does IGI actually measure to assign a cut grade?

For round brilliant diamonds, IGI compares a stone’s overall proportions against its own research into brightness, fire, scintillation, and pattern to produce the cut grade. The five core measurements are total depth, girdle thickness, pavilion depth, crown angle, and table width, all recorded as percentages or degrees on the certificate. Polish (the quality of each facet surface) and symmetry (how precisely the facets align and meet) are graded separately on the same Excellent-to-Poor scale and listed as their own line items below the overall cut grade.

Those three grades together, cut, polish, and symmetry, are what the trade calls Triple Excellent or 3EX. For round brilliants, 3EX is achievable and worth targeting. For fancy shapes such as ovals or cushions, IGI does not issue an overall cut grade at all; it provides only polish and symmetry grades plus a proportions table, so buyers need to evaluate depth percentage and table percentage directly on the certificate.

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 cut grade affect price more than most buyers expect?

Cut is the one C that does not change after the diamond leaves the cutter. Color and clarity are fixed properties of the crystal; cut is a craftsmanship decision, and a bad one cannot be undone. A poorly cut 1.5-carat stone will look smaller and duller than a well-cut 1.2-carat stone, because perceived size in a round brilliant is driven partly by how light exits the table.

Because of this, the market prices cut quality into the stone. An Excellent-cut round lab-grown diamond typically commands a premium over an identical Very Good stone, and that premium is often justified: the Excellent stone will outperform the Very Good in any lighting environment a buyer is likely to encounter. Going below Very Good to save money on cut is one of the few tradeoffs in diamond buying that is almost always visible.

Cut Grade Is the Grade That Pays You Back

Of everything on an IGI certificate, cut grade is the one that determines whether the diamond looks alive or flat on the finger. Color and clarity contribute to value on paper; cut contributes to what every person in the room actually sees. For a round brilliant, targeting Excellent or Ideal, with matching Excellent grades for polish and symmetry, is the clearest way to get a stone that performs at its price point. If you are browsing IGI-certified loose lab-grown diamonds, the cut grade line is the first number worth checking before anything else on the report.

Frequently Asked Questions

Does IGI use the word “Ideal” on every certificate, or only for certain diamonds?

IGI uses the Ideal grade only for round brilliant diamonds that meet the strictest proportion requirements for light performance. Fancy-shaped diamonds such as ovals, pears, and cushions receive polish and symmetry grades but no overall cut grade on the IGI report, so Ideal does not appear on their certificates.

If a diamond has Excellent cut but only Very Good polish, does that matter?

Yes, it can. Polish affects how smoothly light exits each facet surface. A Very Good polish grade on an otherwise Excellent-cut stone is a minor issue, but it can slightly reduce the crispness of reflections. For the best performance, look for Excellent across all three: cut, polish, and symmetry, which is what the trade calls Triple Excellent or 3EX.

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

They can, especially within the Excellent range, because the grade covers a range of acceptable proportion sets rather than one precise target. Two Excellent-cut stones with slightly different crown angles or table percentages may handle light differently. Watching video footage of a specific stone, or viewing it in person, remains the most reliable check beyond the certificate.

Is the cut grade on an IGI lab-grown diamond certificate graded the same way as for a natural diamond?

Yes. IGI applies the same grading methodology to lab-grown diamonds as to natural ones for cut, color, and clarity. The scale, the proportion benchmarks, and the polish and symmetry criteria are identical. IGI pioneered lab-grown diamond grading in 2005 and continues to use the same standards across both categories.

What cut grade should I accept for an engagement ring center stone?

For a round brilliant center stone, Excellent or Ideal is the practical target in 2026, since the price difference between Excellent and Very Good is often smaller than buyers expect, and the visual difference in most lighting conditions is real. For fancy shapes, focus on the depth and table percentages listed on the certificate rather than a cut grade, since IGI does not assign one for those shapes.