How to Read a Diamond Grading Report

A diamond grading report is an independent laboratory’s scientific description of one diamond. Gemmologists at a laboratory such as GIA or IGI examine the stone, measure it and grade it against a fixed standard, then record the results under a unique report number.

A grading report is not a valuation and it never states a price. It is also not a guarantee of the setting or the jewellery around the stone. Think of it as the diamond’s passport: it identifies the stone and records its characteristics, so you can compare one diamond fairly with another.

The first lines identify the stone. Read them before anything else, because they are how you confirm the report and the diamond belong together.

The measurements and the carat weight are the fingerprint. A diamond of the same shape and weight will almost never share the exact same millimetre dimensions.

Next come the grades that drive a diamond’s appearance and value.

For an oval, cushion, pear or other fancy shape, a GIA report gives no overall cut grade. You judge the cut from the proportions, the polish and symmetry and, above all, by looking at the stone.

Polish and symmetry describe the quality of the cutter’s finishing work, each graded from Excellent to Poor. For a fine engagement diamond, aim for Excellent or Very Good on both.

Fluorescence records whether the stone glows under ultraviolet light, from None to Very Strong. It is a description rather than a quality grade, and in many stones it makes no visible difference. Our guides to polish and symmetry and to fluorescence explain both in detail.

A profile drawing shows the diamond’s proportions: the table size and total depth as percentages, the crown and pavilion angles, the girdle thickness and the culet. These numbers explain how the stone handles light.

You do not need to memorise ideal ranges. What matters is that the proportions are balanced. A stone cut too deep can look smaller than its weight suggests; one cut too shallow can look glassy and lose brightness.

For diamonds of one carat and above, a GIA report includes a plot: a map of the stone with its inclusions marked by symbols. Internal characteristics are drawn in red and surface characteristics in green, and a key explains each symbol.

Read the comments line too. It may note features not shown on the plot, such as clouds or additional pinpoints, or record a laser inscription. Comments are where a careful buyer often learns the most.

Most graded diamonds carry the report number, laser-inscribed on the girdle, the thin outer edge of the stone. It is invisible to the eye and can be read with a jeweller’s loupe or a microscope.

Every report can also be checked online. GIA’s Report Check and IGI’s report verification let you enter the number and see the laboratory’s own record. If the weight, measurements and grades match the stone in front of you, the report is genuine and it belongs to that diamond.

A lab-grown diamond’s report states clearly that the stone is laboratory-grown, and it often names the growth method, HPHT or CVD, and whether the stone has been treated after growth. Lab-grown diamonds are usually inscribed to say so on the girdle.

The same lab-grown stone can be described in different ways by different laboratories, so always read which report you are looking at. Our guide to GIA and IGI explains how the two laboratories compare.

The diamonds we propose for a centre stone come with an independent grading report. We read it with you line by line, show you the stone under magnification, and point out exactly where the report’s words meet what you can see.