IGI Diamond Certificate Glossary: Every Term Defined for Non-Expert Buyers

An IGI grading report tells you exactly what you are buying, in precise, standardized language. The problem is that language was written for gemologists, not shoppers. This glossary translates every field on the report into plain English, so you can compare two stones side by side without guessing what “Very Slightly Included” or “Slightly Thick” actually means in practice.

Most buyers scan the 4Cs and ignore the rest of the certificate. That is a mistake, because the terms below the 4Cs, things like culet size, girdle description, and fluorescence, can change how a diamond looks in real light in ways the grade alone will not warn you about: a Strong Blue fluorescence stone, for instance, can look milky under direct sunlight even if it grades VS1. Understanding girdle thickness alone can explain a $400 price gap between two otherwise identical stones.

What do the identification fields at the top of the report mean?

The top of every IGI report carries three fields before you reach any quality grades: the report number, the report date, and the shape and cutting style.

The report number is the diamond’s unique ID. It is often laser-inscribed on the girdle for positive identification. If you are buying a loose stone, you can verify this number on IGI’s website before you hand over any money. The report date tells you when the stone was graded, which matters if you are buying pre-owned and want to know whether the certificate is current. The shape and cutting style field does two jobs: it names the outline (round, oval, cushion) and the facet arrangement (brilliant, step, mixed), which together determine how light moves through the stone.

What do the 4Cs actually mean on the certificate?

Cut, color, clarity, and carat weight are the four grades most buyers already know by name. On an IGI report they appear in a specific order, and each one has more nuance than the letter or number suggests.

Cut

For round brilliants, IGI grades cut on a five-point scale: Excellent, Very Good, Good, Fair, and Poor. The cut grade is a composite score that folds in proportions, symmetry, and polish. A stone graded Excellent in all three sub-categories will reflect more light than a stone graded Very Good in even one of them, and you will see the difference in person.

Color

Color runs from D (colorless) to Z (light yellow or brown). For lab-grown diamonds, IGI uses the same scale as for natural stones. The difference between D and F is invisible to most people without a controlled viewing environment, which is why many buyers land on G or H as a practical sweet spot for white diamonds.

Clarity

Clarity grades describe the number, size, and position of internal inclusions and surface blemishes. The scale runs FL (Flawless), IF (Internally Flawless), VVS1, VVS2, VS1, VS2, SI1, SI2, and then I1 through I3. For most buyers, VS2 and SI1 are the grades worth studying most carefully, because that is where eye-clean stones and stones with visible inclusions start to overlap.

Carat Weight

Carat is a unit of mass, not size. One carat equals 0.2 grams. A well-cut 0.95 ct round can look larger than a poorly cut 1.05 ct round because the poorly cut stone hides weight in depth. The certificate will not tell you this directly, but the table percentage and depth percentage fields will, which is why those numbers matter.

What are the proportion fields and why do they matter?

Below the 4Cs, IGI lists a set of measurements that most buyers skip. These are the fields that separate a stone worth its price from one that is not.

Table % is the width of the flat top facet expressed as a percentage of the total diameter. For a round brilliant, 54 to 60 percent is the range where light return tends to peak. Depth % is the total height of the stone divided by its average diameter. Too deep and the stone looks small for its carat weight; too shallow and light leaks out the bottom. The sweet spot for a round is roughly 59 to 62.5 percent.

Crown angle and pavilion angle describe the tilt of the upper and lower facets. These two numbers interact: a crown angle of 34 to 35 degrees paired with a pavilion angle of 40.6 to 41 degrees is the combination most associated with the “hearts and arrows” light pattern in ideal-cut rounds.

Star length and lower half % are less discussed but still relevant. Star length affects how the stone handles diffused light (cloudy days, indoor lighting), while lower half percentage influences the size and contrast of the light flashes you see when the diamond moves.

What do girdle, culet, and fluorescence mean?

These three terms appear near the bottom of most IGI reports and are frequently misread or ignored.

Girdle is the narrow band around the widest point of the diamond, where the crown meets the pavilion. IGI describes it in two ways: thickness (Extremely Thin through Extremely Thick) and finish (faceted, bruted, or polished). An Extremely Thin girdle is fragile and chips under normal wear. An Extremely Thick girdle hides weight in a place that contributes nothing to the stone’s face-up appearance, so you pay for carat weight you cannot see. Very Thin to Slightly Thick is the practical range most cutters aim for.

Culet is the tiny point or facet at the very bottom of the stone. IGI grades it from None through Very Large. A None or Pointed culet is standard in modern cuts. A Large or Very Large culet creates a visible dark circle when you look straight down through the table, which is why antique cuts like Old European and Old Mine often look different from modern rounds even at the same clarity grade.

Fluorescence describes how the diamond reacts to ultraviolet light. IGI grades it None, Faint, Medium, Strong, and Very Strong, with a color (almost always Blue). None and Faint have no practical effect on appearance. Medium Blue is neutral in most lighting. Strong and Very Strong Blue can make a D-F color stone look hazy in sunlight, though the same fluorescence sometimes improves the appearance of a J or K color stone by counteracting its yellow tint. The effect is stone-specific, which is why fluorescence is one field where seeing the actual diamond matters more than reading the grade.

The Certificate Grade Is a Snapshot, Not the Whole Story

An IGI certificate records every measurable quality of a diamond at the moment of grading. What it cannot record is how those qualities interact in a specific setting, under your lighting conditions, on your hand. The proportions section tells you how light should behave in theory; the actual performance depends on the cutter’s execution across hundreds of individual facets.

That is why the certificate is the starting point, not the finish line. Use it to eliminate stones with deal-breaking flaws (an Extremely Thin girdle, a Large culet, Strong fluorescence on a colorless stone) and to narrow your shortlist to stones worth viewing. Once you are down to two or three candidates, the certificate stops being the most useful tool. Your eyes take over.

For shoppers building from scratch, the Certified Lab Diamonds | Ethical & Stunning collection at Ouros Jewels pairs IGI grading reports with each stone, so the certificate terms above map directly to what you see in the listing.

Frequently Asked Questions

What is the difference between IGI and GIA grading for lab-grown diamonds?

IGI grades lab-grown diamonds using the same 4Cs scale as GIA but has built dedicated lab-grown grading infrastructure and tends to issue reports faster. GIA re-entered the lab-grown market in 2020 but initially used a different descriptor system. As of 2026, both labs use letter grades for lab-grown color and clarity, making their reports broadly comparable for consumer purposes.

Does a higher IGI clarity grade always mean the diamond looks better?

Not always. A VS1 stone with an inclusion near the center table can be more visible than an SI1 stone whose inclusion sits near the girdle, hidden by a prong. The clarity plot on the IGI report shows inclusion positions, and that diagram is worth reading before you assume a higher grade means a cleaner-looking stone.

Can I verify an IGI report number before buying online?

Yes. IGI publishes a report verification tool at its official website where you can enter the report number and confirm the grades match what the seller is listing. This takes about two minutes and is the single most reliable check you can do before purchasing a loose stone or certified piece remotely.

What does “laser inscription” on the IGI report mean for a buyer?

It means the report number has been etched onto the diamond’s girdle using a laser, readable under 10x magnification. When you receive the stone, a jeweler can confirm the inscription matches the certificate number. This rules out stone-switching, which is the main fraud risk when buying certified diamonds through resellers or secondary markets.

Why do two IGI-certified stones with identical 4C grades sometimes have very different prices?

The 4Cs capture four variables, but the proportion fields, fluorescence, and culet grade introduce additional variables that affect desirability. A 1.00 ct G VS1 Excellent round with None fluorescence and ideal proportions will typically price higher than a 1.00 ct G VS1 Excellent round with Strong Blue fluorescence and a depth percentage of 64.5, even though both certificates show the same four letter grades.