Decode an IGI Certificate Number, the Lab-Grown Specific Guide

IGI grades more lab-grown diamonds than any other lab, and it isn't close. If you're trying to reverse engineer a certificate number from a retailer listing, and the stone is lab-grown, the odds are overwhelming that you're dealing with an IGI report.

We built a complete guide to finding a diamond certificate number when a retailer hides it, covering the full logic across all major labs. This post narrows the focus to IGI specifically, because lab-grown buyers face a few quirks that natural diamond buyers don't: a prefix that appears and disappears across retailer databases, a type notation that looks important but isn't, and a high volume of nearly identical stones that makes the final narrowing step harder than it used to be.

Nine digits and a prefix that keeps disappearing

IGI report numbers are nine digits long. For lab-grown stones, IGI introduced an "LG" prefix to distinguish them from natural diamond reports. You'll see numbers formatted as LG followed by nine digits, or sometimes just the nine digits alone.

The inconsistency matters. Some retailers strip the LG prefix when they import inventory into their systems. Others keep it. A few databases store the number both ways for the same stone, which creates duplicate entries or, worse, makes one diamond look like two.

Our matching system handles both formats. We normalise every IGI lab-grown report number by checking for the LG prefix, storing a canonical version, and matching against both the prefixed and unprefixed forms. Paste either version into a search and we'll find the stone. Not every platform does this, which is why some buyers hit dead ends searching for a number they copied straight off a retailer's listing page.

The nine digit number itself is sequential, not encoded. Unlike GIA report numbers (which we cover in our GIA certificate decoder), there's no shape or weight information embedded in the digits. You can't glean anything about the stone from the number alone. It's purely a lookup key.

Why your big lab-grown oval almost certainly says IGI

GIA grades lab-grown diamonds too. But IGI dominates the space, particularly above 2ct and in fancy shapes. Once you get above 4ct in lab-grown ovals, IGI reports outnumber GIA by a wide margin in our database.

Practical reasons explain the split. IGI has been grading lab-grown stones longer, turnaround times are faster for manufacturers, and the cost per report is lower. Manufacturers optimise for margin. When a lab-grown oval already sells for roughly $1,406 on average compared to $7,411 for a natural equivalent (an 81% gap), spending less on certification is a rational choice.

That 81% gap on ovals is actually one of the narrower spreads right now. Marquise cuts sit at nearly 86%, radiant cuts at almost 85%. Rounds are the outlier at 75.6%, reflecting the persistent premium round brilliants command regardless of origin. We track these live in our lab-grown vs natural price comparison.

For cert decoding purposes: if you're looking at a lab-grown stone above 2ct in a fancy shape, assume IGI. You'll be right the vast majority of the time.

Narrowing gets harder when every stone looks the same

Cert decoding works by taking the specifications a retailer publishes and matching them against known certificates until you narrow to a single stone. The logic is the same for lab-grown and natural, with one exception worth understanding.

Most CVD lab-grown diamonds are classified as type IIa on their IGI report. This is a crystallographic designation meaning the stone has negligible nitrogen impurities. It shows up as a scientific note, and some retailers market it as though it adds value.

It doesn't. Type IIa is simply a byproduct of the CVD growth process, not a quality indicator. If a lab-grown stone was made by CVD (and most above 1ct are), it will be type IIa. Don't let a retailer charge extra for it.

For spec narrowing, type IIa is useless as a differentiator because nearly every CVD stone carries the same classification. The filters that actually do the work are the familiar ones: shape, carat weight (to the hundredth), colour grade, clarity grade, and sometimes cut or polish grades.

Lab-grown presents one genuine challenge for spec matching: production volume. Manufacturers produce stones to tight specifications, so clusters of nearly identical inventory are common. Five or six 2.01ct E VVS2 ovals from the same manufacturer, all IGI graded, all in the same table and depth range. Not unusual. When that happens, additional differentiators become essential: exact millimetre measurements, fluorescence, or even the symmetry grade.

One 2ct E VVS2 oval, step by step

Say you're looking at a retailer listing for a 2.01ct E VVS2 oval, lab-grown. The listing shows a grading report from IGI but doesn't publish the report number. You want to verify the cert independently, compare pricing across retailers, or simply confirm the specs are accurate.

The narrowing process works like this:

After six filters, you're typically down to a handful of stones. Sometimes one. Sometimes a dozen if it's a popular configuration. From there, check the secondary measurements the retailer lists against the IGI database. Table and depth percentages are usually enough to reach a unique match. If the retailer publishes millimetre measurements (length x width x depth), that's often a fingerprint on its own.

Cross-retailer pricing data matters here. Lab-grown ovals show an average cross-retailer spread of roughly 48.6%. The gap between the cheapest and most expensive listing for comparable stones is enormous. Match the cert, and you can compare the exact same stone at different retailers to see who's charging a premium.

Lab-grown rounds show a 70.7% average spread across retailers. That means the most expensive listing for a given spec can be nearly three times the cheapest. Cert matching is one of the most direct ways to make sure you're not the buyer paying top dollar.

When two characters go missing

The LG prefix issue deserves its own section because it catches people repeatedly.

IGI issues lab-grown reports with an LG prefix on the certificate itself. But when retailers import inventory through feed systems, CSV uploads, or database migrations, those two characters sometimes get stripped. The diamond enters a retailer's system as a bare nine digit number.

This creates a simple but frustrating problem. You find a report number on one site, search for it on another, and get zero results. Or you paste it into IGI's own verification portal and it doesn't resolve. The fix is obvious once you know about it: add or remove the LG prefix and try again.

Our system treats both forms as equivalent. When we ingest inventory from 100+ retailers, we normalise IGI lab-grown report numbers into a canonical format and match against both variants. A stone appearing on one site with the LG prefix and on another without it still resolves to a single identity in our database.

This matters because if you can't match across retailers, you can't spot the pricing gaps. And those gaps are real. A lab-grown round showing up at two different price points with a spread of over 70% represents the same stone (or a functionally identical one) priced by retailers with very different margin strategies.

Matching is getting harder, and that's fine

Lab-grown production continues to scale, and with it, the number of nearly identical stones in circulation keeps climbing. Matching a 2ct E VVS2 oval to a single IGI report sometimes requires more secondary measurements than it did a year ago, simply because more stones hit those exact specifications now.

The counterbalance: retailers are listing more detail. Table percentage, depth percentage, crown angle data. These fields appear more frequently on lab-grown listings than they used to, and every additional data point makes spec matching more reliable even as production volume rises.

If you want to skip the manual process, our cert decoder handles the matching automatically across the full index. Either way, confirming the cert independently before you buy is one of the few genuinely free advantages available to diamond buyers. Especially when lab-grown cross-retailer spreads on rounds alone average over 70%.