How to read a diamond grading report, line by line

A diamond grading report is an independent laboratory's description of a single diamond: what it weighs, how it measures and how it grades. You read it by confirming identity first, through the report number and any girdle inscription, then carat weight and the grades for colour, clarity and cut, then the finish grades for polish and symmetry, with the fluorescence and the plot of the stone's clarity characteristics last.

This guide follows a GIA or IGI report roughly in the order its lines appear, saying what each one measures and which lines change the look and the price. Which laboratory to choose is a separate question, covered in our comparison of GIA and IGI reports.

What is a diamond grading report, and is it a certificate?

Most people say certificate. The laboratories generally call the document a report, which is more accurate: it records what trained graders observed and measured on the day the stone was examined. It carries no guarantee.

It does not give a price either. Two stones with identical grades can sell for different amounts, and an insurer will usually ask for a separate valuation. And because the report describes the stone as it was when graded, a diamond since chipped or recut is no longer quite the stone on the paper.

Start with identity: the report number and girdle inscription

Before reading any grade, check that the report belongs to the diamond. Every report carries a unique number, and the laboratory keeps a record against it. Many graded diamonds also carry that number inscribed by laser on the girdle, the thin edge around the widest part of the stone, too small to read without a 10x loupe.

If the report lists an inscription, the stone should show it, character for character. Not every diamond is inscribed, and an inscription can be polished away or added to a different stone, so it is only a first check. Identity is settled when the inscription, the weight, the measurements and, where the report has one, the clarity plot all agree.

Shape, cutting style and measurements

The next lines describe the outline and facet arrangement, in phrases such as round brilliant, oval modified brilliant or emerald cut. Measurements follow in millimetres: for a round, the smallest and largest diameter and the depth; for other shapes, length, width and depth.

These figures tell you more about how large a stone will look than its weight does, because two stones of the same weight can spread differently across the face. For an oval, pear or marquise, length divided by width gives the ratio that makes one oval look slender and another almost round.

Carat weight, colour and clarity

These are the lines most people know, and our guide to the 4Cs of diamonds covers them in depth.

Carat weight is recorded to two decimal places, and one carat is a fifth of a gram. Prices tend to step up at round figures such as half a carat and one carat, so a stone just under one of those marks can look the same on the hand and cost less. Our page on diamond carat weight explains how weight becomes visible size.

Colour is graded from D, which is colourless, to Z, which shows a light yellow or brown tint. Neighbouring grades are very hard to tell apart once a diamond is face up in a ring; our guide to diamond colour grades explains where the steps become visible.

Clarity describes internal features, called inclusions, and surface features, called blemishes, as seen under 10x magnification. The scale runs from Flawless and Internally Flawless through VVS1 and VVS2, VS1 and VS2, SI1 and SI2, down to I1, I2 and I3. Once the stone is in a ring, the test is whether anything shows without magnification, and our guide to diamond clarity explores it step by step.

Cut grade, and why fancy shapes read differently

On a round brilliant, the cut grade is the line most likely to change how the stone looks. GIA states that its overall cut grade applies to standard round brilliants in the D-to-Z colour range. It runs Excellent, Very Good, Good, Fair and Poor, and weighs brightness, fire and scintillation alongside weight ratio, durability, polish and symmetry.

So a GIA report on an oval, a pear or an emerald cut carries no overall cut grade for now, though GIA has said it plans to add one for ovals, pears and marquises in 2027. IGI has published its own system for grading cut on fancy shapes, so an IGI report on the same shape may carry one. Either way, on a fancy shape you judge cut from the proportions, the polish and the symmetry, as our guide to diamond cut explains, and above all from images and video of the stone.

Beside the grades sits a proportions diagram: table and depth percentages, crown and pavilion angles, girdle thickness and culet. On a fancy shape it is most of what the report can tell you about cut. Glance at the girdle on any shape: a very thin one is more exposed to chipping, and a very thick one carries weight where it does not show.

What do polish and symmetry mean on a diamond report?

Polish is the quality of the surface finish on the facets: how smooth they are, and whether polishing left fine lines or marks. Symmetry is how exactly the facets are shaped, placed and aligned, and how true the outline is. Both are graded from Excellent to Poor, on rounds and fancy shapes alike.

Between Excellent and Very Good, the difference is rarely visible without magnification. Below Very Good, ask why, particularly for symmetry, since misaligned facets can disturb the pattern of light across the stone.

Fluorescence, and when it matters

Fluorescence records whether the diamond glows under long-wave ultraviolet light. On a GIA report it reads None, Faint, Medium, Strong or Very Strong, with the colour of the glow, usually blue, named from Medium upwards. GIA's own research found that for the average observer blue fluorescence made no consistent difference to appearance and that strongly fluorescent stones were sometimes judged to look whiter face up. It also found that a hazy look does occur but is rare. In the higher colour grades strong fluorescence is often priced lower, which can make it worth considering. Our guide to diamond fluorescence explains whether you can see it and when it matters.

Reading the plot, the key to symbols and the comments

The plot is a map of the stone's clarity characteristics, drawn over an outline of the diamond from above and below. On a GIA report, red symbols mark internal characteristics and green or black symbols mark surface ones, and the key to symbols names each type: crystal, feather, cloud, needle, pinpoint and so on. The diagram is approximate, so read it for type and position rather than exact size.

Not every report has a plot. GIA's Diamond Dossier, a shorter report for natural diamonds from 0.15 to 1.99 carats, leaves it out and lists the main clarity characteristics instead; the report number is inscribed on the girdle.

Position matters as much as the grade. A small feather near the girdle can sometimes sit under a claw; a dark crystal beneath the table sits where the eye goes first. A feather that reaches the surface needs a closer look, since it can be a point of weakness.

The comments line notes characteristics the plot does not show, such as clouds or pinpoints, and it is one of the places a laboratory discloses any treatment it has detected.

Does the report say whether a diamond is natural or laboratory-grown?

Yes. GIA and IGI both issue a separate, clearly titled document for laboratory-grown diamonds, and the two now read differently. Since October 2025, GIA's quality assessment for colourless and near-colourless laboratory-grown diamonds describes each stone as Premium or Standard rather than giving it individual colour and clarity grades, and the girdle is inscribed Laboratory-Grown beside the number. IGI's laboratory-grown reports still carry the full set of grades, and the inscription commonly carries the report number with the letters LG; their comments commonly state the growth method, CVD or HPHT, and whether there is any indication of treatment after growth.

Both are real diamonds. They differ in origin, and with it in price; our pages on natural diamonds and laboratory-grown diamonds set out the rest.

How do you check a diamond report is genuine?

Forged reports exist, and so do genuine reports paired with a different stone. Both are caught the same way: check the document against the laboratory's record, then the record against the stone.

1. Open the laboratory's own website, checking that the address is gia.edu or igi.org, and enter the report number: GIA Report Check for a GIA report, or IGI Verify Your Report for an IGI one.
2. Compare every line of the online record with the document in your hand: shape, measurements, carat weight, colour, clarity and, where given, cut. They should be identical.
3. Where the report lists an inscription, find it on the girdle under a 10x loupe.
4. Have the loose stone weighed and measured; a clear difference from the report is a reason to stop.
5. Under magnification, compare the stone's inclusions with the plot, or with the characteristics the report lists.

Printed reports usually carry security features such as a hologram, but the laboratory's database is the stronger check. Do all of this while the stone is loose: once set, it can no longer be weighed, and parts of it are hidden by metal.

Which lines should decide your stone?

For a round brilliant, begin with the cut grade, then weigh colour and clarity together, then carat weight. For a fancy shape, the proportions, ratio, polish, symmetry and images of the stone stand in for the cut grade. For every shape, read the plot, or the listed clarity characteristics, rather than the clarity grade alone.

The lines that rarely decide anything on their own are the culet, a girdle within the normal range and fluorescence at None or Faint. Our article on the 4Cs and a fifth worth adding names that fifth as confidence: understanding what the report in front of you says.

Reading a report with us

We keep no loose stock to browse. We source natural and laboratory-grown diamonds for each commission, so a report is usually how you first meet a stone. A GIA or IGI grading report is available on centre stones, and we talk you through each report alongside images and video of the stone, so you can see which grades are shaping the price. If the stone is where you would like to begin, you can start with a diamond and have the ring built around it.

Our studio is inside The Goldsmiths' Centre, at 42 Britton Street in London, and works by appointment. Consultations take place there, by phone or by WhatsApp, and we typically reply within four working hours, Monday to Friday. CAD design is from £300. The fee is non-refundable, and it is credited to the commission if you go ahead. When you approve the CAD design, a 50% deposit is due, less the fee already paid, and it is non-refundable once production begins. The balance is payable before collection or delivery. Every ring we make above the legal exemption weight is hallmarked at the London Assay Office, which speaks for the metal as the report speaks for the stone. Book a consultation and we will read the reports with you.

Common questions

Is a grading report the same as a valuation?

No. A grading report describes the stone and its grades; it does not give a price. For insurance you will usually need a separate valuation.

How do I check a GIA or IGI report number?

Enter it on the laboratory's own website and confirm that the online record matches your document line by line.

Does every diamond have a laser inscription?

No. Many graded diamonds carry one, but not all, and an inscription alone does not prove identity. The weight, the measurements and, where the report has one, the plot should agree as well.

Does a GIA report give a cut grade for an oval diamond?

Not yet. GIA's overall cut grade applies to standard round brilliants in the D-to-Z colour range. For an oval, judge cut from the proportions, polish, symmetry and images of the stone.

Does a GIA report on a laboratory-grown diamond give colour and clarity grades?

Not since October 2025. GIA now describes colourless and near-colourless laboratory-grown diamonds as Premium or Standard. IGI's laboratory-grown reports still give individual colour and clarity grades.