IGI Certification Explained: What Every Section of the Report Means for Lab Diamond Buyers

An IGI grading report is the official document that verifies a lab-grown diamond’s identity, quality, and origin. It records the 4Cs cut, color, clarity, and carat weight , the growth method (CVD or HPHT), fluorescence, polish, symmetry, and a laser inscription number that ties the physical stone to its paperwork. Knowing what each section means puts you in control of the comparison rather than trusting a sales pitch.

Most buyers glance at the color and clarity grades, then stop. That’s understandable, but it leaves a lot of useful data on the table. The sections you skim past, such as the proportions diagram and the comments field, can tell you things the headline grades cannot. This guide walks through the report from top to bottom so you know exactly what you’re reading before you buy.

What does the top section of an IGI report tell you?

The header block is where the report establishes the stone’s identity. It records the report date, a unique report number, the shape and cutting style (for example, “Round Brilliant” or “Oval Modified Brilliant”), and the stone’s precise measurements in millimeters. For a round diamond, you’ll see minimum diameter by maximum diameter by depth. For a fancy shape like a radiant or oval, you’ll see length by width by depth.

That report number matters beyond filing. Every IGI report can be verified at igi.org using the number or the QR code printed on the certificate. Before completing any purchase, it takes about 30 seconds to confirm the report is genuine and that the grades on screen match the grades on the actual certificate in the IGI database.

How should you read the 4Cs section?

This is where most buyers spend most of their time, and with good reason. Carat weight, color, clarity, and cut are the four grades that drive both appearance and price.

Carat weight is recorded to the nearest hundredth of a carat. One carat equals 0.20 grams. Worth remembering: carat measures mass, not diameter. A well-proportioned 1.50-carat stone will often look larger face-up than a poorly cut 2.00-carat stone, because cut controls how the weight is distributed.

Color is graded on the D-to-Z scale, where D is colorless and Z carries a visible yellow or brown tint. IGI gemologists assess color with the diamond placed upside down and viewed through the side, a technique that removes the distraction of sparkle and produces a neutral read. Multiple graders submit independent opinions; the final grade is set when enough assessors agree. For lab-grown diamonds, D through F is colorless, G through J is near-colorless.

Clarity is assessed at 10x magnification on a scale from Flawless down to Included (I3). The report also includes a clarity diagram, a schematic map showing where inclusions sit inside the stone. That diagram is worth studying. Two diamonds can share a VS1 grade but have inclusions in very different positions; one placed near the table will be more visible than one tucked near the girdle.

Cut is where the report becomes most detailed for round brilliants. IGI assesses overall proportions against its own research into brightness, fire, scintillation, and pattern to produce a grade ranging from Excellent to Poor. For fancy shapes like ovals, cushions, and emeralds, IGI does not assign a single overall cut grade, but it does record polish and symmetry grades for every shape.

What do the proportions diagram and finish grades mean?

Below the 4Cs, the proportions section shows a cross-section diagram of the stone with key measurements labeled: table percentage, depth percentage, crown angle, pavilion angle, girdle thickness, and culet size. These numbers explain why two Excellent-cut diamonds can still look different face-up.

A table percentage between roughly 54% and 58% on a round brilliant tends to maximize light return. A pavilion angle between 40.6 and 41.0 degrees produces the strongest brilliance. If you’re comparing two stones with the same cut grade, the proportions diagram is the tiebreaker.

Polish grades the quality of the surface finish on each facet, from Excellent to Poor. Symmetry grades how precisely the facets align and meet. Both are graded on the same five-point scale. Stones graded Excellent in both categories are sometimes marketed as “Ex/Ex” and tend to command a small premium, though Very Good in either category is functionally indistinguishable to the naked eye.

What does the comments section disclose, and why does it matter?

The comments field at the bottom of the report is easy to overlook, but it carries disclosures that affect the stone’s value and your decision. Two things appear here that do not appear anywhere else on the report.

First, the growth method: CVD (Chemical Vapor Deposition) or HPHT (High Pressure High Temperature). Both methods produce diamonds that are chemically, physically, and optically identical to mined diamonds. Neither is inherently better than the other, but the disclosure matters because some CVD diamonds undergo post-growth color treatment, and IGI will note that in the comments if it applies. “As grown with no indication of post-growth treatment” is the language you want to see.

Second, fluorescence: graded as None, Faint, Medium, Strong, or Very Strong, with the color noted (typically blue). Strong fluorescence in daylight can occasionally make a diamond appear milky, though most buyers will never notice it. Medium or Faint fluorescence is rarely a concern and sometimes makes a near-colorless stone look slightly whiter in sunlight.

Why do most lab diamond sellers choose IGI over GIA?

IGI was founded in 1975 and began certifying lab-grown diamonds in 2005, years before most of the industry took them seriously. That early commitment built a body of experience and calibration data specific to lab-grown stones that GIA, which focused primarily on natural diamonds for decades, has not yet matched at scale.

Today, IGI certifies the large majority of lab-grown diamonds sold by major online retailers and independent jewelers. GIA can certify lab-grown diamonds, but it is significantly more expensive and rarely chosen for them. The cost difference is better invested in a higher clarity or color grade on the stone itself. For lab-grown diamonds, where pricing is driven by specifications rather than resale expectations, that trade-off makes practical sense.

One calibration difference worth knowing: IGI grades tend to run slightly more lenient than GIA on color and clarity, roughly one grade on average. A diamond graded VS1/G by IGI might receive VS2/H from GIA. This is not a flaw; it reflects how each lab calibrates its master stones. Buyers who account for it tend to target VVS2 or higher clarity and E color or higher on IGI reports, which closes the gap and ensures the stone is genuinely eye-clean and white in any metal setting.

The IGI Report Is a Tool, Not Just a Trophy

Reading an IGI grading report well means treating it as a comparison instrument, not a marketing document. The report number verifies authenticity. The 4Cs set the quality baseline. The proportions diagram tells you how the stone will actually perform in light. The comments section discloses everything the headline grades do not.

If you are shopping for IGI-certified lab-grown diamonds and want stones where every section of the report has already been vetted, Ouros Jewels lists certified lab diamonds with full IGI documentation available before purchase. Use the report number at igi.org to confirm the grades independently, then compare the proportions diagram across stones at the same grade. That process, not the certificate alone, is what leads to a confident buy.

Frequently Asked Questions

How do I verify that an IGI report is real and hasn’t been altered?

Enter the report number printed on the certificate directly at igi.org. The database will display the grades, stone measurements, and clarity diagram on file. If anything on the physical certificate differs from what the database shows, the document has been altered. This check takes under a minute and should be done before any purchase.

Does the IGI report tell me whether a lab diamond has been treated after it was grown?

Yes. The comments section explicitly states whether the stone was grown with no post-growth treatment or whether treatment was applied. CVD diamonds sometimes require color correction after growth; IGI discloses this. An untreated stone will read “as grown with no indication of post-growth treatment,” which is the preferred language for buyers who want full transparency.

Is a laser inscription on a lab-grown diamond standard, or do I have to request it?

Most IGI-certified lab-grown diamonds carry a laser inscription on the girdle, the outer edge of the stone, that matches the report number exactly. It is invisible to the naked eye but readable under magnification. This inscription permanently links the physical stone to its certificate, making it much harder to swap stones after purchase. If a stone you are considering does not have one, ask the seller why.

What IGI cut grade should I aim for in a round brilliant lab diamond?

Excellent is the standard target for round brilliants, and most well-priced lab-grown rounds on the market in 2026 already carry it. The more useful question is whether the proportions behind that grade are strong: look for a table between 54% and 58%, a depth between 61% and 63%, and a pavilion angle between 40.6 and 41.0 degrees. Two Excellent-graded stones with different proportions will not sparkle identically.

Can I use an IGI report to insure my lab-grown diamond?

An IGI grading report documents the stone’s characteristics but is not an appraisal. Most insurers require a separate appraisal document that assigns a replacement value in dollars. Your jeweler or an independent appraiser can produce one using the IGI report as the quality reference. Keep both documents together and update the appraisal if lab-grown diamond prices shift significantly.