How to read a diamond certificate in 90 seconds

The grading report is the only independent statement of fact in the entire transaction. Everything else on a product page is written by someone who wants to sell you the stone.

It is also the one thing almost nobody checks, because it looks technical. It is not. There are about six things on it that matter and you can get through them in a minute and a half.

First, a terminology point

The trade says “certificate” and so does everyone else, but the GIA calls them reports, deliberately. A certificate implies a guarantee of value. A grading report describes characteristics and says nothing about what the stone is worth.

An appraisal is a different document again: it estimates replacement value for insurance. It is not evidence of quality and it is usually written by someone connected to the sale.

So: the report tells you what the stone is. The appraisal tells you what it would cost to replace. They are not interchangeable and only one of them is independent.

Step one: verify the number

Before reading anything, check the report exists.

Every report has a number. Both laboratories publish free lookups on their own sites: GIA at gia.edu, IGI at igi.org. Enter the number from the listing and the laboratory returns its own record.

Confirm two things. That a report with that number exists, and that it says what the retailer’s page says it says. Carat, colour, clarity, cut grade, shape, measurements should all match exactly.

This takes fifteen seconds and it is the single most valuable step in the whole process. A retailer misrepresenting a stone is rare among the established names, but transcription errors are not, and you are about to spend several thousand pounds.

Most stones above about half a carat also carry the report number laser-inscribed on the girdle, the narrow edge round the stone’s widest point. It is invisible without magnification but a jeweller can confirm it matches. Worth doing at the point of insurance appraisal.

Step two: which laboratory

Grading standards vary between laboratories, and a grade means nothing except in the context of who issued it.

Stick to GIA, IGI or GCAL. GIA is the benchmark for natural stones and generally the strictest. IGI grades the majority of lab-grown diamonds and is the standard there. GCAL appears at some retailers and is well regarded.

A report from a laboratory you have not heard of is not necessarily fraudulent, but it may be graded more generously, which means a G on that report might be an H or I on a GIA one. If a stone looks unusually cheap for its stated grades, check who graded it before assuming you have found a bargain.

If there is no report at all, walk away. Smaller accent stones in a setting are not individually graded and that is normal, but a centre stone without a report is a centre stone nobody wanted to document.

Step three: the six things that matter

Read them in this order, because that is the order of how much they affect what you see.

Cut grade. For a round brilliant, this is the single most important line on the report. Excellent or Ideal, nothing less. It describes the workmanship, and workmanship is what makes a diamond look like a diamond.

An important exception: the GIA only assigns a cut grade to round brilliants. Ovals, pears, emeralds, cushions and every other fancy shape get no cut grade at all. If you are buying a fancy shape you cannot outsource this judgement to the report, and you have to look at the proportions and at the stone itself instead. Most guides do not mention this and it catches people out.

Carat weight. Stated to two decimal places. Check it against the price, and remember that weights just under the round numbers cost meaningfully less for a near-identical stone.

Colour. D through Z. What matters is the metal you are setting it in: G or H is the sweet spot for white gold and platinum, and you can drop two or three grades in yellow or rose gold with no visible difference, because the metal is warm anyway.

Clarity, and the plot. The grade matters less than where the inclusions are. The report includes a plotted diagram showing their position, and position is what determines visibility. An inclusion under the table, the large flat facet on top, is far more visible than one near the girdle where the setting may hide it entirely.

Two stones graded SI1 can look completely different for this reason. Look at the plot, then look at the stone’s photograph at magnification, and decide with your own eyes whether it is eye-clean.

Polish and symmetry. Two separate lines, both graded on the same scale as cut. Very Good or better on each. These are cheap to get right and there is no reason to accept less.

Step four: the ones people misread

Fluorescence. Some diamonds glow under ultraviolet light. The report grades it from None to Very Strong.

In most stones this is invisible in normal conditions and completely irrelevant. In a small number, strong fluorescence causes a faint haziness. Because the market treats fluorescence as a defect, stones carrying it are often discounted — which means a Medium or Strong fluorescence stone with good images can be a genuine saving rather than a compromise. Look at the pictures and judge.

The proportions block. Table percentage, depth percentage, crown and pavilion angles. For a round brilliant with an Excellent cut grade you can ignore these; the grade already accounts for them.

For fancy shapes, where there is no cut grade, these are what you have. As rough guidance, a depth percentage far outside the high 50s to low 60s, or a table percentage above the low 60s, is worth asking about. But proportions for fancy shapes are genuinely a matter of taste as well as optics, so use the images as the final arbiter.

Measurements. Length by width by depth, in millimetres. Useful because carat is weight, not size: two stones of identical weight can present differently on the hand depending on how the weight is distributed. For fancy shapes, the length-to-width ratio is what determines whether an oval looks long and elegant or short and round, and that is entirely a preference.

Lab-grown certificates are now different

This changed recently and a lot of guidance has not caught up.

In late 2025 the GIA stopped grading lab-grown diamonds on the same scale as natural ones. Rather than individual colour and clarity grades, lab-grown stones now receive descriptive categories: Premium and Standard.

The reasoning is that when more than 85% of lab-grown stones grade in the top few colour ranges, the distinctions stop carrying useful information.

What this means for you: a lab-grown GIA report and a natural GIA report no longer describe the stone in the same terms, and comparing them grade for grade does not work the way it used to. Lab stones graded by IGI may still carry conventional grades, as may older GIA-graded stones. Check which system you are actually looking at before comparing two stones.

The 90-second version

1. Look up the report number on the laboratory’s own site and confirm it matches the listing.
2. Check it is GIA, IGI or GCAL.
3. Cut grade Excellent or Ideal, if it is a round. If it is a fancy shape, there is no cut grade, so look at the stone.
4. Colour matched to the metal.
5. Clarity eye-clean, judged from the plot and the photograph rather than the grade.
6. Polish and symmetry Very Good or better.

If all six check out, the report has told you everything it can. The rest is whether you like the look of it.

This article was last checked in September 2026. Grading standards and laboratory practices change; confirm current details with the laboratory directly. It contains affiliate links: if you buy through them we may earn a commission at no extra cost to you.