Diamond certification provides independent verification of quality, protecting buyers from misrepresentation. It is also one of the most misunderstood documents in the jewellery trade. A grading report is a precise, narrow piece of work: it describes a stone, and nothing else. Knowing where it stops is what turns it from a reassuring sheet of paper into something you can use.
What a grading report is, and what it is not
A grading report is a laboratory’s opinion of a diamond’s characteristics, recorded against standardised scales by trained graders who work without knowing who owns the stone. It states weight, measurements, proportions, colour, clarity, finish, fluorescence and any treatment the laboratory has detected. It ties that description to a unique report number.
What it is not is a valuation. A grading report carries no price, no estimate of worth and no insurance figure, and a document that does carry one is an appraisal, produced by a valuer rather than a laboratory, and answering a different question entirely. Nor is a report a guarantee. Colour and clarity are judgements made by people to a defined standard, not readings from an instrument, and two competent graders can land on adjacent grades for a borderline stone.
Read this way, a report is a common language. It lets you compare two stones you cannot hold at once, and describe your diamond years later to someone who has never seen it.
The major grading laboratories
GIA
The Gemological Institute of America is the reference point for the industry. It devised the 4 Cs framework and the colour and clarity scales that almost everyone else now uses, and its reports are recognised worldwide. GIA is a non-profit and grades stones without buying or selling them, which is the root of its reputation for independence.
GIA is generally regarded as strict and, more importantly, consistent: the same stone submitted twice is likely to come back the same. Its cut grade for round brilliants runs Excellent to Poor, and the round brilliant is currently the only shape that receives an overall cut grade on a GIA report.
One recent change matters. Since October 2025 GIA has stopped applying natural-diamond 4 Cs nomenclature to laboratory-grown stones, using descriptive quality terms instead of letter and number grades on its laboratory-grown reports. If you are comparing a grown stone graded before that date with one graded after, you are reading two different systems.
AGS
The American Gem Society built the other well-known cut grading system, scoring cut on a numerical scale on which zero is the top grade rather than using words, and it was long the laboratory of choice for buyers who cared most about light performance.
AGS Laboratories closed at the end of 2022, and GIA acquired its technology and intellectual property. AGS reports still in circulation are perfectly readable, but they are historical documents. GIA now issues an AGS Ideal report as a supplementary document for qualifying stones, though it applies GIA’s own light performance measures rather than the old AGS methodology.
IGI
The International Gemological Institute was founded in Antwerp in 1975 and is now headquartered in Mumbai. It is the largest laboratory by volume, with a particularly strong position in laboratory-grown diamonds, and it offers grading, identification and laser inscription services across a wide network of locations.
IGI reports are widely accepted, and for laboratory-grown stones they are often the only reports you will encounter. The trade’s general view is that IGI grading has historically been a little more generous than GIA on colour and clarity, particularly on smaller stones, so a grade from one laboratory should not be treated as interchangeable with the same grade from another.
HRD Antwerp
HRD Antwerp is the Belgian laboratory, based in the Antwerp diamond district with facilities in Dubai and Mumbai. It grades both natural and laboratory-grown diamonds, assesses cut, and offers a separate Hearts and Arrows assessment for precision-cut round brilliants. HRD is well established in Europe and less familiar to buyers elsewhere, which is a recognition issue rather than a quality one.
Others, and the ones to treat carefully
Plenty of smaller laboratories issue reports, and a jeweller’s own in-house document is not a laboratory report at all. The cautionary tale is EGL: in 2014 the RapNet trading network delisted diamonds accompanied by EGL reports over concerns about inconsistent grading, and EGL International subsequently wound down. Older EGL papers still turn up in second-hand and estate stock. They are not worthless, but treat the grades on them as a starting point rather than a description.
Why the same stone can be graded differently
Colour is assessed by comparing a stone against a set of masterstones under controlled lighting. Clarity is assessed by a trained eye at a defined magnification, deciding not only how visible an inclusion is but where it sits and what it does to the stone. Both are careful, repeatable human judgements — but they are human judgements, and laboratories differ in how they resolve the borderline case.
The consequence is practical. Two diamonds listed with the same colour and clarity from two different laboratories are not necessarily the same diamond, and any comparison you make between them is only as sound as the assumption that the two laboratories grade alike. When you compare, compare like with like: GIA against GIA, IGI against IGI. Where you must cross between laboratories, treat the grades as approximate and let the stone itself settle it.
Reading a report section by section
Most reports follow the same order, whichever laboratory issued them.
- Identification: the report number and date, and the type of report. The date matters because grading standards and report formats change.
- Shape and cutting style: for example round brilliant, or rectangular step cut.
- Measurements: millimetre dimensions and depth. On a round brilliant these are given as minimum and maximum diameter plus depth, and a wide gap between the two diameters tells you the stone is not truly round.
- Carat weight, given to two decimal places.
- Grading results: colour and clarity, and for a round brilliant the cut grade.
- Finish: polish and symmetry, graded separately.
- Fluorescence: whether the stone glows under ultraviolet light, and how strongly. This is a description, not a fault.
- Proportions diagram: a profile drawing with table percentage, crown and pavilion angles, girdle description and culet. This is where cut quality actually lives, and we cover how to read it in our guide to cut.
The plot
The plot is the outline diagram, shown crown-up and pavilion-up, with the stone’s clarity characteristics marked on it: red for internal features such as crystals, feathers and clouds, green or black for surface features such as naturals and extra facets. A key beside it names each symbol.
The plot is the closest thing a diamond has to a fingerprint. No two stones carry the same arrangement of inclusions, which is why the plot is the tool you use to confirm that the stone in front of you is the stone the report describes. Ask to see it under magnification and match a distinctive feature to the diagram. Note that laboratories generally plot only the characteristics needed to establish the grade, so a plot is not an exhaustive inventory.
The comments section
Read this before anything else. The comments carry the disclosures, and a stone’s whole story can sit in two lines of small print. Expect to find, where applicable:
- Treatments: clarity enhancement such as laser drilling or fracture filling, and colour treatment such as irradiation or high pressure high temperature processing. Reputable laboratories disclose these, and some decline to issue a full grading report on certain treated stones at all.
- Origin: whether the diamond is laboratory-grown. This is always stated, and on modern reports it is stated prominently and inscribed on the stone as well.
- Laser inscription: the text engraved on the girdle, quoted so you can match it.
- Clarity characteristics not shown on the plot, and notes such as internal graining or surface graining.
What an inscription is, and why it helps
A laser inscription is microscopic text on the girdle, the narrow band around the widest part of the stone. It usually carries the report number, sometimes with the laboratory’s name, and it is invisible without magnification and does not affect the grade.
Its value is identification. An inscribed stone can be matched to its report in seconds by a jeweller with a loupe, a genuine comfort when a diamond is left for resizing or repair. Not every graded stone is inscribed, and its absence is not a warning sign, but if the report says an inscription exists, ask to see it.
Verifying the number against the laboratory’s own database
This is the single most useful thing in this article, and it takes about a minute.
Every major laboratory maintains a free online archive of the reports it has issued. GIA has Report Check, IGI has its own report verification lookup, and HRD Antwerp provides access to its report archive. You enter the report number, sometimes with the carat weight, and the laboratory returns the details it holds. Compare those details, line by line, against the document in your hand.
Do this even when the paper looks impeccable, because the paper is the easy part to fake. The two failure modes it catches are a report that never existed, and a genuine report number attached to a different, lesser stone. The database is the only version that counts; the printed report is a copy of it.
Warning signs
- The report number does not appear in the laboratory’s database, or the details returned do not match.
- The document is a photocopy, a photograph or a scan, and the seller cannot produce the original or a verifiable digital report.
- The issuing body is a name you cannot find independently, or is the seller’s own business under another name.
- Grading language is borrowed from the recognised scales without naming an independent laboratory — phrases such as store-graded or appraised as.
- The comments section is missing, or the report has no plot on a stone large enough to warrant one.
- The stone in front of you does not match the report: wrong measurements, wrong weight, or a plot whose distinctive inclusion you cannot find.
- The seller discourages you from checking the number, or wants a decision before you can.
None of these is proof of bad faith on its own. Any of them is reason to slow down.
Where this leaves you
Ask which laboratory graded the stone, and prefer one whose consistency you can rely on. Read the comments before the grades. Verify the number in the laboratory’s own database yourself, on your own device. Then put the paper down and look at the diamond, because the report describes the stone but it was never meant to replace it.