What GIA & IGI Certification Means When Buying a Diamond

If you're spending £1,000+ on a diamond, you should be buying a stone with an independent grading report. That report is the only objective record of the diamond's quality — and it's the document a future jeweller, valuer or insurer will use to confirm what you actually own.

The two laboratories you'll see most often in the UK are GIA (Gemological Institute of America) and IGI (International Gemological Institute). Both are reputable. They have slightly different positions in the market, slightly different reports, and they're sometimes priced differently. Here's what you need to know.

What a grading report actually contains

Whichever lab issues the report, the contents are similar. Every full grading report includes:

• A unique report number (you can verify it on the lab's website)
• Shape and cutting style (e.g. round brilliant, oval brilliant, emerald cut)
• Measurements (length, width, depth in millimetres)
• Carat weight (exact, to two decimal places)
• Colour grade (D–Z scale)
• Clarity grade (FL through I3)
• Cut grade (Excellent, Very Good, Good, Fair, Poor — for round brilliants; sometimes for fancy shapes)
• Polish and Symmetry grades
• Fluorescence (None, Faint, Medium, Strong, Very Strong)
• A plot diagram showing the position of inclusions (on full reports)
• Proportions diagram showing critical angles
• Laser inscription confirmation (most modern stones have the report number microscopically inscribed on the girdle)

The diagram showing inclusion positions is the part most people skip but shouldn't — if the inclusions are dead-centre, an SI clarity stone can be a worse buy than a VS2 stone of equal price.

GIA — the gold standard

The Gemological Institute of America is the laboratory most often cited as the gold standard for diamond grading. It is the lab that originally devised the 4C grading system in the 1950s, and it has the strictest grading standards of any major lab.

A few key points about GIA:

• Founded 1931 in California. Non-profit. Independent of any retailer.
• Strictest grading. A "G colour, VS1 clarity" stone graded by GIA is, on average, a touch more conservative than the same grade from other labs.
• Most often used for natural diamonds. GIA grades both natural and lab-grown, but the natural diamond market is overwhelmingly GIA-graded.
• Reports are universally accepted by jewellers, valuers and insurance companies worldwide.
• Verifiable at gia.edu by report number.

Because GIA grades conservatively, a "GIA G/VS1" stone often commands a small premium over an "IGI G/VS1" of the same dimensions — but the difference is rarely more than 5–10% and is widely considered fair.

IGI — increasingly mainstream, especially for lab-grown

The International Gemological Institute is the second major laboratory you'll see. It is the dominant lab for lab-grown diamonds globally and is widely used for natural diamonds in jewellery retail.

Key points about IGI:

• Founded 1975 in Antwerp, Belgium. Multiple labs worldwide.
• Major lab for lab-grown diamonds. If you're buying lab-grown, you'll see IGI reports more often than GIA — IGI's process was earlier to scale for lab-grown grading.
• Grading is comparable but slightly less conservative than GIA in some categories. The difference is small but real, and is one reason IGI stones often retail for slightly less than GIA-graded equivalents.
• Reports are accepted by mainstream insurance companies and valuers.
• Verifiable at igi.org by report number.

For lab-grown diamonds in the UK, IGI certification is the norm and is completely fine. For high-end natural diamonds (over £10,000), most buyers prefer GIA.

What about other labs?

You may also see:

• HRD (Belgium) — credible, sometimes graded slightly less strictly than GIA.
• AGS (American Gem Society) — also strict; in 2023 AGS merged its grading operations into GIA, so it is now effectively part of GIA.
• EGL (European Gemological Laboratory) — historically less strict; "EGL G/VS1" is widely understood to be one or two grades less conservative than "GIA G/VS1". Avoid if possible — the price discount usually isn't worth the inconsistent grading.
• In-house jeweller certificates — these are not independent. They are the jeweller's own grading of their own stone. Never accept these as a substitute for an independent lab report.

The simple rule: insist on GIA or IGI. Don't accept EGL or in-house grading.

How to verify a report is real

Every GIA and IGI report has a unique number. Type that number into gia.edu/report-check or igi.org/verify and you'll see the report on the lab's own website, with all the grading details. If a "certificate" you've been given doesn't verify on the issuing lab's website, it isn't genuine.

A laser inscription on the diamond's girdle (visible at 10x magnification) is also a strong authenticity check — the report number is microscopically engraved on the stone, so the report and stone can be cross-referenced.

What the report doesn't tell you

A grading report tells you what a stone is. It doesn't tell you what the stone looks like. Two G/VS1 round brilliants graded by GIA with identical paper specs can look noticeably different on the finger because of subtle differences in cut quality, light performance, and the position of inclusions.

This is why a good jeweller will always show you the stone before you commit, ideally next to a comparable stone of similar specs but slightly different price. The certificate tells you what you're paying for; the stone in front of you tells you whether you actually want it.

What we do at Diamond Hub

Every diamond we sell at Diamond Hub is graded by either GIA or IGI. We supply the report number with every quote, and the original report ships with the finished ring. You can verify the report at the issuing lab's website before you buy.

For natural diamonds, we use GIA by default. For lab-grown diamonds, we use IGI (which is the industry standard for lab-grown grading). On rare occasions we will use HRD or AGS — and we'll always explain why if we do.

If you'd like to see how the same paper specs can look on different stones, book a free showroom consultation — we'll lay out two or three stones of identical grading specs and let you see why cut quality, polish and symmetry actually matter beyond what a grade alone tells you.

A note on insurance valuations

A diamond grading report is not an insurance valuation. The report tells you what the stone is; the valuation tells you what it would cost to replace. Most insurance companies will accept an independent valuation from any registered UK jeweller (NAJ members in particular). Every Diamond Hub piece ships with an independent insurance valuation as standard.

Further reading

• The 4Cs of Diamonds Explained
• Diamond Shapes Guide
• Official: GIA Report Check and IGI Verification