Diamond Certification Guide: GIA & IGI Explained

A diamond certificate is the one piece of paperwork in this entire process that actually matters, it’s the independent, third-party record of what you’re buying, separate from anything the seller tells you. Every diamond we work with is certified by GIA or IGI, and this guide explains what each report actually says, how they differ and how to read one with confidence.
This is part of our full Engagement Ring Guide. It pairs directly with our Colour Guide and Clarity Guide, since the grade on your certificate is what those guides are built around.

What a Certificate Actually Is
A grading report (commonly called a “certificate,” though the labs themselves use “report”) is an independent assessment carried out by a gemological laboratory with no financial stake in the sale. A gemologist examines the stone under controlled conditions and records its characteristics — carat weight, cut quality, colour, clarity, and measurements — against a standardised scale.

The point of this is comparability. Without a third-party report, you have no way to verify that a stone described as “colourless, eye-clean” actually is, or how it compares to another stone described the same way by a different seller.

GIA: The Long-Established Authority
The Gemological Institute of America developed the 4Cs system and the D-to-Z colour scale that the entire industry still uses — for natural diamonds, GIA remains the strictest and most widely recognised name in grading, and its reports carry the strongest weight for resale and insurance purposes.

An important recent change for lab-grown stones. As of October 2025, GIA no longer issues detailed colour and clarity grades for lab-grown diamonds. Instead of a specific letter and clarity grade, lab-grown stones submitted to GIA now receive one of two classifications:
• Premium — requires D colour, VVS clarity or higher, and excellent finish (with an excellent cut grade required for round brilliants)
• Standard — covers E–J colour and VS clarity, with very good finish
Stones that don’t meet the minimum bar for Standard receive no GIA grade at all. GIA made this change because, in its assessment, the overwhelming majority of lab-grown diamonds fall within a narrow band of colour and clarity, making the fine distinctions used for natural diamonds — which vary far more, given their geological origins — less meaningful for lab-grown stones.

The practical effect: a GIA-certified lab-grown diamond tells you which broad tier it sits in, but not the specific colour letter or clarity grade within that tier.

IGI: The Lab-Grown Specialist
The International Gemological Institute has certified lab-grown diamonds for longer, at greater volume, than any other major lab, and continues to issue full detailed grading for them — specific colour letters (D through the alphabet) and specific clarity grades (FL through I3), exactly as it does for natural diamonds.

If the level of detail in our Colour and Clarity guides matters to you choosing between an I and a J colour for a yellow gold setting, for instance, or deciding between VS2 and a well-selected SI1 an IGI report is what gives you that specific information to act on.

IGI’s grading is generally considered to run slightly more lenient than GIA’s, by roughly half a grade to a full grade, depending on the characteristic. This isn’t a sign of an unreliable report it’s a different lab applying its own consistent standard — but it’s worth knowing when comparing stones across labs rather than within one.

Which to Look For, Depending on What You Want
If you want the specific colour and clarity detail covered in our other guides, to make a precise, informed choice against your metal and setting, look for an IGI report.

If you want the reassurance of GIA’s name specifically, and you’re comfortable working within the Premium/Standard framework rather than a precise letter grade, a GIA-certified stone is a genuinely reliable choice, it simply groups quality slightly more broadly.

Either way, both labs are independent, reputable, and verifiable this isn’t a question of one being trustworthy and the other not. It’s a question of how much granular detail you want on the document itself.

Other Labs You Might See
GCAL (Gem Certification & Assurance Lab) and HRD Antwerp also grade diamonds, including lab-grown stones, using detailed 4Cs-style reporting. Both are reputable, though far less commonly encountered in the UK market than GIA or IGI.

EGL (European Gemological Laboratory) is worth a specific note of caution: it has a well-documented reputation for inconsistent grading, often inflating colour and clarity grades relative to GIA or IGI. A diamond priced attractively against an EGL certificate is not necessarily a better deal — it may simply be a lower-quality stone graded generously.

How to Read a Certificate

Whichever lab issued it, a report will typically include:
• Report/certificate number: unique to that stone, and verifiable directly on the issuing lab’s website
• Shape and measurements: the physical dimensions, not just the carat weight
• Carat weight
• Colour grade (or Premium/Standard classification, for GIA lab-grown reports)
• Clarity grade (or Premium/Standard classification, for GIA lab-grown reports)
• Cut grade: how well the stone is proportioned, separate from shape
• Polish and symmetry: finer measures of how precisely the stone was finished
• Fluorescence: whether the stone shows any glow under UV light, which can very occasionally affect appearance.

It’s always worth verifying a certificate number directly on the lab’s own website (GIA and IGI both offer free report-check tools) before completing a purchase, rather than relying on the physical document alone.

Continue Reading
• Engagement Ring Guide: the full pillar guide covering shape, setting, and metal
• Diamond Colour Guide
• Diamond Clarity Guide
• Lab-Grown vs Natural Diamonds
• Book a Consultation: talk through certification and grading together, before you buy