How to Read a Diamond Grading Report (And What "Certified" Really Means)

Somewhere between "I like that one" and "we'll take it," someone slides a folded document across the counter. It's a diamond grading report, and for most people it may as well be in another language — a grid of abbreviations, a diagram covered in tiny red symbols, and a long number at the top.

That document is the most useful thing you'll be handed while diamond shopping, because it's the only part of the transaction written by someone with nothing to sell you. Here's how to read it. We're Chapel Hills Jewelry, a family-owned jeweler on Freeport Road in the Aspinwall area of Pittsburgh, and we've been having this conversation across the counter since 1989.

What a grading report is — and what it isn't

A grading report is an independent laboratory's description of a specific stone: its measurements, its grades, and anything unusual about it. Well-known laboratories include the GIA (Gemological Institute of America), IGI (International Gemological Institute) and AGS (American Gem Society).

Two things a grading report is not:

• It is not a valuation. A grading report contains no price. It describes the stone; what it's worth is a separate question.
• It is not an appraisal. An appraisal is a valuation document, usually written for insurance purposes, and it is not the same thing as a resale figure. We'll come back to this.

It's also worth knowing that laboratories differ in how strictly they grade. Two reports showing the same colour and clarity grade from two different labs do not necessarily describe equally graded stones. That is the honest answer, and it's the reason a diamond that looks like a bargain on paper sometimes isn't.

Reading the report, line by line

Report number and date

Every report has a unique number and an issue date. The number is the part that matters most — see the verification section below.

Shape and cutting style

For example "Round Brilliant" or "Rectangular Modified Brilliant" (which is how a radiant cut is usually described). Lab language and counter language don't always match, which surprises people. Our diamond shape guide translates between the two.

Measurements

Three numbers in millimetres — the stone's actual dimensions. These tell you how large the diamond will look face-up, which carat weight alone does not. Two one-carat diamonds can differ noticeably in spread depending on how they're cut. For fancy shapes, dividing length by width gives you the ratio that determines whether an oval reads slender or round.

Carat weight

Weight, not size, recorded to the hundredth of a carat. Our carat weight guide covers why the two aren't interchangeable.

Colour grade

A letter from D (colourless) down through the alphabet as warmth increases. Where a stone falls on that scale matters less in isolation than in context — the metal it's set in and the shape both affect how colour reads. See our colour guide.

Clarity grade

A description of the stone's internal and surface characteristics, from Flawless through the Included grades. The grade alone doesn't tell you whether an inclusion is visible or where it sits — that's what the plotting diagram is for. Our clarity guide goes deeper.

Cut grade — the one to read first

On round brilliants, laboratories assign an overall cut grade based on proportions and how the stone handles light. Cut is the grade that most affects whether a diamond actually looks alive, which is why we tell people to read it before colour or clarity. Note that many fancy shapes do not receive an overall cut grade at all — for those you're reading proportions, polish and symmetry instead, and looking at the stone. Our cut guide explains why this grade carries the most weight.

Polish and symmetry

Two separate grades covering surface finish and how precisely the facets align. They're easy to skim past, and they contribute to how crisply a diamond returns light.

Fluorescence

Whether the stone glows under ultraviolet light, and how strongly. It is a characteristic, not a defect — plenty of beautiful diamonds fluoresce. Strong fluorescence occasionally affects appearance in certain stones, which is a reason to look at a diamond in person rather than judging it from a line on paper.

The plotting diagram and key to symbols

The little map of the stone showing where its clarity characteristics sit. This is where "SI1" becomes meaningful: an inclusion under a crown facet in a place a prong will cover is a very different proposition from one sitting in the middle of the table.

Comments and inscription

Read the comments section. Treatments, clarity enhancements, and other notable findings are disclosed here. Many stones also carry a laser inscription of the report number on the girdle, readable under magnification — that's the physical link between the paper and the diamond.

Verify the report yourself

Every major laboratory publishes a free report-check tool on its own website. Enter the report number and you should see the same stone described back to you. If you're buying a graded diamond, do this — it takes a minute, it costs nothing, and it confirms that the document in your hand matches the laboratory's own records. Ask to see the inscription under a loupe or microscope while you're at it; any jeweller should be glad to show you.

Why "certified" deserves a follow-up question

"Certified" gets used loosely in jewellery advertising, and it isn't a regulated shorthand. The only meaningful version of the claim names the laboratory: a diamond accompanied by a GIA report, or an IGI report, or an AGS report. If a stone is described as "certified" with no laboratory named and no report number to check, the right response is simply to ask which lab, and to ask to see the report.

Not every diamond needs one, either. Small accent stones in a pavé band or a halo are not individually graded, and no one expects them to be — grading reports are for the centre stone and other significant diamonds.

Lab-grown stones get reports too

Laboratory-grown diamonds are graded and reported in much the same way, and their reports state clearly that the stone is laboratory-grown. That disclosure is not optional and should be plain on the face of the document. If you're weighing the two, our guide to lab-grown vs natural diamonds lays out the honest differences. Natural diamonds are our primary business; we source lab-grown stones by request.

Grading report vs. appraisal

These get confused constantly, so plainly:

• A grading report comes from a gemological laboratory and describes what the stone is. No value.
• An appraisal is a valuation document, prepared for insurance and identification purposes. It is not a statement of what you could sell the piece for, and the two figures are not the same thing.

If you own fine jewellery, insurance is the practical reason to have an appraisal on file — our appraisals page covers what we do in house, and our article on insuring an engagement ring covers the next step.

Read the report, then look at the diamond

A grading report tells you what a stone is. It cannot tell you whether you like it. We've watched people choose the lower grade on paper more than once, because in daylight, on a hand, it was the better-looking diamond — and that's a completely legitimate way to decide.

The useful thing is to do both: read the paperwork so you know exactly what you're comparing, then look at the stones side by side. We're glad to sit down and go through a report with you line by line, whether it's a diamond you're considering here or one you're weighing somewhere else. See our loose diamonds, learn how we build a ring around a stone on our custom engagement rings page, or stop by 503 Freeport Road, Pittsburgh, PA 15215. You can also call 412-782-1757.