How to Read an IGI Diamond Certificate: A Section-by-Section Guide for Buyers

An IGI lab grown diamond grading report is a standardized document that records a stone’s measurable characteristics: carat weight, color grade, clarity grade, cut grade, proportions, polish, symmetry, fluorescence, growth method, and laser inscription number. Reading it correctly tells you whether the diamond you’re buying matches what’s on paper and whether its price is justified. Most buyers scan the 4Cs and stop there. That leaves several fields unread, and some of those fields carry real consequences for how the stone looks and what it’s worth.

The report itself is printed on yellow paper for lab grown diamonds (white for natural stones), and every field follows a consistent layout across IGI’s global network of labs. Once you know what each section does, you can compare any two IGI-certified stones in about three minutes.

What do the report header fields actually tell you?

The top of every IGI report carries three pieces of identification: the report date, the report number, and the shape and cutting style. None of these affect the diamond’s beauty, but all three matter for verification.

The report number is the stone’s permanent ID. IGI’s LASERSCRIBE system engraves this number microscopically on the girdle of every lab grown diamond it certifies, along with the words “LAB GROWN.” You can read it under a 10x jeweler’s loupe, and you can verify it free at igi.org in under a minute. If the number on the stone doesn’t match the number on the paper, stop the transaction.

Below the report number, the shape and cutting style field distinguishes the outline (round, oval, cushion) from the faceting pattern (brilliant, step, mixed). An octagonal stone, for instance, could be a radiant cut or an emerald cut, two very different performers in terms of light return. The measurements line below that gives the stone’s dimensions in millimeters: minimum and maximum width, then depth. A 1-carat round diamond might measure anywhere from 6.2 mm to 6.5 mm across depending on how it was cut, which is worth knowing before you set it.

How do the 4Cs appear on the report, and which one matters most?

The center of the report lists carat weight, color grade, clarity grade, and cut grade. These four grades drive roughly 90 percent of a diamond’s price, so this is where most buyers spend their time.

Carat weight is recorded to the nearest hundredth of a carat. One carat equals 0.20 grams. The key thing to remember: carat measures mass, not visual size. A well-proportioned 1.50-carat stone will often face up larger than a poorly cut 2.00-carat stone, because spread depends on how the weight is distributed, not just how much there is.

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 under standardized lighting, a method designed to eliminate the visual distraction of sparkle and produce a neutral reading. Multiple graders submit independent assessments, and the final grade is set when enough opinions agree. For lab grown diamonds, D through F is colorless; G through J is near-colorless. Most buyers targeting value land in the G-to-I range, where color is barely perceptible to the naked eye but prices are noticeably lower than D-F.

Clarity is assessed at 10x magnification. The scale runs from Flawless (FL) at the top down to Included (I3), where characteristics are visible without magnification. VS1 and VS2 are the practical sweet spot for most buyers: inclusions exist but are invisible to the naked eye and don’t threaten the stone’s structure. Lab grown diamonds can carry inclusions just as natural diamonds do, though the type sometimes differs by growth method. CVD stones may show faint graining or cloud-like formations; HPHT stones occasionally contain metallic flux inclusions. Neither disqualifies a stone, but they’re worth knowing about.

Cut grade governs how much light the stone returns to the eye, expressed through brightness, fire, and the diamond’s apparent size when worn. For round brilliants, IGI compares the stone’s proportions against its own studies of brightness, fire, scintillation, and pattern to arrive at a grade from Excellent-Ideal down to Poor. For fancy shapes (ovals, cushions, pears, emeralds), IGI uses a four-step system assessing finish, proportions, shape-specific requirements, and light return, but does not issue a single overall cut grade. If you’re buying a round, Excellent or Very Good cut is the practical minimum for a stone that genuinely sparkles.

What do the finish grades and proportions diagram tell you?

Below the 4Cs, the report lists polish, symmetry, and fluorescence, followed by a proportions diagram.

Polish reflects how smooth the diamond’s surface is after cutting. Symmetry grades how precisely the facets align and meet. Both run from Excellent to Poor. When both achieve Excellent alongside an Excellent cut, the diamond is sometimes called “Triple Excellent,” a standard that buyers in 2026 increasingly seek out for round brilliants because it correlates with the crispest light return.

The proportions diagram is a side-profile drawing of the stone annotated with table percentage, depth percentage, crown angle, and pavilion angle. These numbers let you predict how light travels through the diamond. A table percentage between 54% and 58% and a crown angle near 34 to 35 degrees, combined with a pavilion angle of 40.6 to 41.0 degrees, tends to produce the best balance of brightness and fire in a round brilliant. The diagram makes these comparisons possible without a gemologist in the room.

Fluorescence records the stone’s reaction to ultraviolet light, graded from None to Very Strong, with color noted (usually blue). Strong fluorescence occasionally makes a high-color diamond look slightly hazy in direct sunlight, though in lower color grades (I, J) faint blue fluorescence can actually make the stone appear whiter. It’s a nuance worth noting, not a reason to reject a stone outright.

What does the clarity plot show, and how do you read it?

The clarity plot is a diagram of the diamond’s shape, annotated with symbols showing the location and type of every inclusion and blemish identified during grading. Red symbols indicate internal characteristics (inclusions); green marks indicate surface characteristics (blemishes).

Common inclusion types you’ll see noted include pinpoints (tiny crystal formations), feathers (small fractures), and clouds (clusters of pinpoints). The plot functions as a fingerprint: no two diamonds have identical inclusion patterns in identical locations, which makes the plot a useful tool for confirming that the stone in your ring is the same one that was graded.

One practical note: some IGI reports omit a full legend for the symbols and state only that red means internal and green means external. If you want to identify a specific inclusion type, cross-reference the Comments field, which sometimes names significant characteristics.

What does the Comments section disclose about lab grown diamonds?

The Comments section is the most commonly skipped field on the report. For lab grown diamonds, a buyer who skips it may not know their stone was color-treated post-growth, which can affect resale value by 20 to 30 percent compared to an untreated stone of the same grade.

This is where IGI discloses the growth method: either CVD (Chemical Vapor Deposition) or HPHT (High Pressure High Temperature). These are the two processes used to grow lab diamonds, and both produce stones that are chemically, physically, and optically identical to mined diamonds. Neither CVD nor HPHT produces a better diamond on optical or durability grounds. What matters is that the method is disclosed, and the IGI report provides that disclosure as standard.

The Comments section also flags any post-growth treatments. Some lab grown diamonds undergo color treatments after production that can affect long-term value. If the Comments field reads “None” or is blank (indicating no treatments), that’s the outcome most buyers want. If it notes a treatment, that’s not automatically a disqualifier, but you should factor it into the price conversation.

The Clarity Plot and Inscription Are What Protect Your Purchase

Every other field on an IGI report describes the diamond. The clarity plot and the laser inscription are the two fields that verify it is your diamond.

The inscription number, engraved on the girdle in characters visible only under magnification, links the physical stone to its paper record. When a diamond arrives set in a ring, you or your jeweler can check the girdle under a loupe, confirm the inscription matches the report number, and close the loop on authenticity. Without it, stone substitution is possible. With it, the chain of custody is documented.

At Ouros Jewels, every lab grown diamond comes with a full IGI grading report rather than a condensed card certificate. The card format covers only the basic 4Cs and the inscription number, with no proportions data, no clarity plot, and no treatment disclosure. A diamond with excellent color and clarity can still be a poor performer if its cut is mediocre, and a card certificate won’t tell you that. The full report will.

Start with cut grade, which governs how much light the stone returns; move to color and clarity, which set the price ceiling at a given appearance level; finish with the Comments section, where growth method and treatment disclosures live. Then cross-check the pavilion angle against the 40.6 to 41.0 degree target and confirm the clarity plot symbols match the Comments field.

Frequently Asked Questions

What is the difference between an IGI full report and an IGI card certificate for a lab grown diamond?

The full IGI grading report includes all 4Cs, the proportions diagram, polish and symmetry grades, fluorescence, growth method disclosure, the clarity plot, and a Comments section with any treatment disclosure. The card certificate covers only the basic 4Cs and the inscription number, with no proportions data, no clarity plot, and no treatment disclosure. For any purchase over a few hundred dollars, the full report is the only document that gives you a complete picture.

How do I verify that a diamond matches its IGI certificate before I buy?

Go to igi.org, enter the report number in the verification tool, and confirm the grades match the listing exactly. Then, when you receive the diamond, ask your jeweler to read the laser inscription on the girdle under a 10x loupe and confirm it matches the report number. This two-step check takes under five minutes and closes any gap between what was graded and what was shipped.

Does the growth method (CVD vs. HPHT) on the IGI report affect diamond quality or value?

Neither CVD nor HPHT produces a superior diamond in terms of optical performance or durability. Both methods yield stones that are chemically and physically identical to mined diamonds. The growth method is disclosed in the Comments section primarily for transparency. What matters more to value is the 4Cs and whether any post-growth treatments are noted alongside the growth method disclosure.

Why does cut grade matter more than carat weight when reading an IGI report?

Cut grade governs how the diamond returns light, which controls brightness, fire, and how large the stone appears face-up. 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. Carat weight sets the price per unit; cut determines what the buyer actually sees.

What does fluorescence on an IGI certificate mean for how a diamond looks?

Fluorescence measures the diamond’s glow under ultraviolet light, graded from None to Very Strong. In most lighting conditions, fluorescence has no visible effect on appearance. Strong fluorescence can occasionally make a high-color (D-F) diamond look slightly hazy in direct sunlight. In lower color grades like I or J, faint blue fluorescence can make the stone appear slightly whiter. It is a secondary consideration, not a primary quality indicator.