How to read an IGI diamond certificate

If you're buying a lab grown diamond, the stone should come with a certificate from an independent grading lab. For lab grown diamonds, the most common is IGI (International Gemological Institute). For natural diamonds it's usually GIA. Both labs do similar work, but for the lab grown market IGI dominates.

Here's how to read what's on the certificate and what to actually pay attention to.

What an IGI certificate is

It's an independent assessment of a diamond's characteristics. The diamond goes to the lab, trained gemmologists examine it under standardised conditions, and the lab issues a report with the stone's specifications.

The certificate isn't proof of value (the lab doesn't tell you what the stone is worth). It's a description of what the stone is. The pricing and value comes from comparing the certification to the price you're being asked to pay.

The information on the certificate

An IGI report includes:

• Report number. A unique number, often laser-inscribed onto the stone itself. You can verify the certificate by entering this number on the IGI website.
• Stone type. Lab grown diamond (sometimes worded as "laboratory-grown diamond"). For lab stones, the growth method (CVD or HPHT) is usually noted.
• Shape and cutting style. Round brilliant, oval modified brilliant, princess, etc.
• Measurements. Length, width, and depth in millimetres. For round stones, also the table percentage and depth percentage.
• Carat weight. The diamond's weight, to two decimal places.
• Colour grade. From D (colourless) to Z (light yellow). For engagement rings, D to H is generally desirable.
• Clarity grade. From FL (flawless) to I3 (heavily included). VS1 and VS2 are typical for engagement rings.
• Cut grade. Excellent, Very Good, Good, Fair, Poor. Only applies to round brilliants; fancy shapes don't get a cut grade.
• Polish and symmetry. Graded separately, both should ideally be Excellent.
• Fluorescence. None, Faint, Medium, Strong, Very Strong. Most engagement ring buyers want None or Faint.
• Inclusion plot. A diagram showing the location, type, and approximate size of inclusions inside the stone.

What to focus on

The grades that matter most:

Cut grade. Most important for round brilliants. An Excellent cut grade means the stone has been cut to optimal proportions for light return. A poorly cut stone with great colour and clarity will look duller than a well-cut stone with mediocre colour and clarity. If you're choosing a round brilliant, only buy stones with Excellent cut.

Colour. D to F are colourless and significantly more expensive. G to H are near-colourless and look identical to colourless in most settings. I and below start to show a faint warm tint, especially in yellow gold settings. For most rings, G or H gives you the best value.

Clarity. Covered in detail in our clarity guide. Short version: VS1 or VS2 is the sweet spot for engagement rings.

Polish and symmetry. Both should be Excellent or Very Good. Anything lower can affect how the stone catches light.

What's less important

Fluorescence. The marketing pitch is that fluorescence makes diamonds appear cloudy. In reality, only Strong or Very Strong fluorescence noticeably affects appearance, and even then only in certain lighting. None or Faint fluorescence is fine.

The inclusion plot. Worth looking at, but don't obsess. The clarity grade already summarises whether the stone is eye-clean. Use the plot to check whether inclusions are central (more visible) or peripheral (less visible).

Specific measurements. The table and depth percentages matter for cut quality, but the cut grade already factors them in. Unless you're optimising for a specific look, the grade is sufficient.

How to verify a certificate

Go to igi.org and enter the report number. The site will pull up the certificate as it was issued. If the certificate you've been shown doesn't match the official IGI record, that's a problem.

You can also ask to see the laser inscription on the stone itself. Reputable jewellers will show you the inscription under magnification on request.

IGI vs GIA

Common question. For lab grown diamonds, IGI is the standard and is what most lab diamond manufacturers and retailers use. For natural diamonds, GIA is the historical standard.

Both labs use the same grading scales (the GIA scale, originally) and apply similar standards. There's been historical debate about whether IGI grades slightly more loosely than GIA, but for lab grown diamonds the comparison is less relevant because GIA only started grading lab stones recently.

For a lab grown diamond, an IGI certificate is the standard and is fully trustworthy.

The honest take on certification

Don't buy a diamond without certification. The certificate is what lets you compare stones across different jewellers on a like-for-like basis. A jeweller who can't show you a certificate (or who downplays its importance) is asking you to take their word on the stone's specifications.

But also: the certificate is a starting point, not the whole story. Two stones with identical certificates can look quite different in person depending on the specific characteristics, and a well-cut stone with a slightly lower grade can outperform a poorly cut stone with higher grades.

If you want help interpreting a specific certificate or want to compare certified stones in person, that's part of what we do in a free consultation.

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