Cut Grade for Lab-Grown Diamonds: Ideal vs Excellent Explained

Cut grade accounts for up to 40% of a lab grown diamond's perceived brightness, more than color and clarity combined, making it the one variable that controls how much light the stone actually returns to your eye. A D-color, VVS1 stone with a Fair cut will look noticeably duller than an H-color, VS2 stone with an Excellent cut, because the difference is geometric: a diamond's facet proportions either redirect light back through the crown, or let it leak out the sides and base.

This guide covers what the grade scale actually means, whether "Ideal" and "Excellent" (and "Triple Excellent") are really different things, the exact proportions to target, and how to judge cut on fancy shapes that don't get a formal grade at all.

What brilliance, fire, and scintillation actually mean

A well-cut diamond produces three distinct optical effects simultaneously:

• Brilliance: the white light a diamond reflects back to the eye, from internal reflections bouncing between pavilion facets and returning through the table.
• Fire: dispersion, the splitting of white light into spectral colors as it bends through the crown, producing flashes of red, orange, and blue.
• Scintillation: the pattern of bright and dark flashes that appears as the stone, viewer, or light source moves.

These effects depend on diamond's physical constants (a fixed refractive index of 2.42 and dispersion of 0.044) and cut quality, not on whether the stone was grown in a lab or mined. A lab grown diamond with an Excellent cut performs identically to a mined diamond with the same proportions.

The six cut grades, ranked

For a round brilliant, grades run Ideal, Excellent, Very Good, Good, Fair, Poor. Each step down is a measurable reduction in how much light the stone returns.

GIA uses this same five-tier scale (Excellent through Poor) for round brilliants; IGI, the dominant certifier for lab grown diamonds today, uses an equivalent scale but labels its top tier "Excellent-Ideal." A separate system from the American Gem Society runs 0-to-10, where 0 is Ideal and is generally stricter than GIA Excellent. For most buyers shopping IGI-certified lab grown stones, Ideal and Excellent are the two grades worth prioritizing; everything below Very Good shows trade-offs no color or clarity upgrade can fix.

Is "Triple Excellent" different from "Ideal"?

Almost certainly not visibly. "Triple Excellent" (3EX) is industry shorthand for a diamond earning Excellent in all three measurable categories (cut, polish, symmetry), not a single official grade line. "Ideal" has a different origin: it was the American Gem Society's top tier, and IGI now uses a combined "Excellent-Ideal" designation for its highest round-brilliant grade. GIA uses "Excellent" as its ceiling, with no "Ideal" tier above it. In practice, the grade name tells you which lab issued the certificate more than it tells you how the diamond performs.

GIA Triple Excellent vs IGI Ideal, by the numbers

IGI's Ideal tier applies tighter proportion windows than GIA's Excellent range: a diamond at the wide edge of GIA's Excellent tolerance can earn Triple Excellent, while that same stone might only grade Excellent (not Ideal) at IGI. That's why two "Triple Excellent" stones from different labs can perform differently in light, and why checking the actual proportion numbers matters more than the grade label. The real visible variation happens within a grade, not between Excellent and Ideal.

The Ideal label's premium (typically a few percentage points on top of the 10-15% that Triple Excellent already commands over Very Good) is worth paying mainly when buying online without seeing the stone, where the tighter IGI window acts as a useful filter. If you can review HD video or have a gemologist inspect the stone, the standard proportion numbers are usually enough.

The exact proportions to target (round brilliant)

The grade label is a starting point, not a finish line: two diamonds can both carry "Excellent" and perform differently. Request the full proportion data from the certificate and check it against these targets:

• Table percentage: 54-57%. Too wide (above 60%) suppresses fire because light exits straight back through the top. Too narrow (below 53%) can create a dark, windowed look.
• Total depth: 61-62.5%. Below 59%, light leaks through a too-shallow pavilion. Above 62.5%, the stone starts looking smaller than its carat weight suggests, because excess weight hides under the setting.
• Crown angle: 34-35°. Governs fire, the spectral color flashes.
• Pavilion angle: 40.6-41°. Governs brilliance. Above 41°, the diamond can develop a dark "nail head" ring under the table; below 40.6°, light leaks through the bottom.

Crown and pavilion angle don't work independently: a 34.5° crown paired with a 40.8° pavilion produces a different result than the same crown paired with a 41.4° pavilion, even if both fall within the "Excellent" range. The pairing that most consistently produces top performance is a 34-34.8° crown with a 40.8-41° pavilion. Polish and symmetry should both read Excellent; the culet should be none or pointed; the girdle thin to slightly thick and faceted.

Cut grade on fancy shapes: no formal grade, read this instead

GIA and IGI don't issue an overall cut grade for ovals, pears, cushions, emeralds, radiants, or marquise: light behavior in those shapes can't be reliably predicted from proportions alone the way it can for a round. (IGI introduced an optional fancy-shape cut grade in late 2022, but it's not mandatory, so most reports in 2026 still omit it; its absence is normal, not a red flag.) Instead, evaluate four things directly:

• Depth percentage, oval: 57-62%; cushion/radiant: 67-70%; pear/marquise: 58-62%; emerald: tends higher, step-cut geometry runs differently.
• Table percentage, oval: 53-64%; radiant: 64-68%; emerald: 61-69%.
• Length-to-width ratio, oval: 1.35-1.50; marquise: 1.85-2.05 (acceptable 1.75-2.25); pear: 1.40-1.60.
• Polish and symmetry: target Excellent in both; IGI's own fancy-shape system requires a minimum of Very Good in each before a stone can even be considered for its optional Excellent cut grade.

These ranges are filters, not guarantees: a stone within range still needs to be seen. That's especially true for the bow-tie effect: a dark shadow across the center of elongated shapes (ovals, pears, marquise most visibly), caused by pavilion facets that fail to redirect light back to the viewer. A faint bow-tie is normal and expected; a heavy, persistent one is a sign of poor facet execution that no proportion number will warn you about. The only reliable check is a 360-degree HD video under standard lighting; proportions tell you where to start, video tells you where to stop.

Does cut grade matter more than color or clarity?

For light performance, yes. Lab grown diamonds are produced under controlled conditions, so color and clarity defects are less common than in mined stones; most lab grown diamonds on the market fall in the D-to-G color range and VS2-or-better clarity without much extra spend. Cut is different: a poorly proportioned stone wastes every other quality attribute. You can pay for D color and VVS1 clarity and still end up with a stone that looks flat under office lighting because the pavilion is leaking light. A well-cut H-color stone in a simple setting will catch light from across a room. Upgrading color on a Fair or Good cut stone produces no visible improvement; cut builds the ceiling; color and clarity just set the floor.

The bottom line

For a round brilliant, target Ideal or Excellent: table 54-57%, depth 61-62.5%, crown angle 34-35°, pavilion angle 40.6-41°, with Excellent polish and symmetry. Very Good is a defensible trade-off if the savings are significant and the stone sits in a setting with its own surrounding sparkle; Good, Fair, and Poor are grades to avoid for a center stone. For fancy shapes, check depth, table, and length-to-width ratio against the shape-specific ranges above, confirm Excellent polish and symmetry, and always request video to judge bow-tie severity before buying. Ouros Jewels sources IGI-certified lab grown diamonds and provides the full grading data so these proportions can be verified directly before purchase.

Frequently Asked Questions

What's the best cut grade for a lab grown diamond engagement ring?

Ideal or Excellent for a round brilliant. Very Good is acceptable if budget requires a trade-off; anything below that shows light leakage no setting can compensate for.

Is IGI Ideal the same as GIA Excellent (or Triple Excellent) for lab grown diamonds?

They're equivalent in intent but not identical in criteria: IGI's Ideal tier uses tighter proportion windows than GIA's Excellent range. The more reliable comparison is the actual proportion numbers, not the grade name.

Can a fancy-shaped lab grown diamond get an Ideal or Excellent cut grade?

Generally no formal overall grade; only polish and symmetry are standard, with IGI's optional fancy-shape cut grade still uncommon on 2026 reports. Evaluate depth, table, and length-to-width ratio against the shape-specific targets above instead.

Will a Good cut grade save meaningful money versus Excellent?

Typically only 5-10% for the same carat, color, and clarity, rarely enough to justify the visible trade-off. Dropping a color grade (F to H) or a clarity grade (VVS2 to VS2) usually saves more money with less visible impact than dropping cut grade.

What causes the bow-tie effect, and is it a sign of poor cutting?

A dark shadow across elongated shapes caused by facets that don't redirect light back to the eye in that zone. A faint bow-tie is normal and not a defect; a heavy, persistent one signals poor facet execution. Since no lab formally grades bow-tie severity on most reports, video is the only reliable check.

Does cut grade affect lab grown diamonds the same way it affects mined diamonds?

Yes: diamond's optical properties are fixed by its carbon crystal structure, not by how it formed. An Excellent-cut lab grown diamond reflects light identically to a mined diamond with the same proportions.