Most diamonds come with a grading report, with one of the most widely recognised being a GIA report. These reports provide valuable information about a diamond, documenting its characteristics following an assessment by an independent gemmological laboratory.
A grading report is a useful tool for understanding a diamond's characteristics and helping to assess its quality and value. However, while it provides a great deal of important information, there are also some limitations to what it can tell you.
In this guide, we'll use a GIA grading report as our example, as it is one of the most commonly used diamond reports.
Report Number
The first piece of information you'll find on a GIA report is a unique report number. On most modern diamonds, this unique number will also be laser inscribed on the girdle of the diamond (the narrow edge around the diamond). The inscription is only visible under magnification, but it allows the diamond to be matched to its corresponding grading report and is a unique identifier for that particular stone.
Shape, Cutting Style and Measurements
A GIA report identifies the diamond's shape and cutting style, such as round brilliant, oval brilliant, step cut or modified brilliant. It also records the diamond's measurements, including its length, width and depth, in millimetres.
It's quite normal for a round diamond to have a very slight variation between its length and width. A difference of around 0.03mm is generally nothing to be concerned about.
Carat Weight
The carat weight of a diamond is recorded to two decimal places on a GIA grading report. It’s important to remember that carat is a measurement of weight, not size. A diamond can carry more of its weight in its depth, for example, rather than across its surface. This means that two diamonds with the same carat weight can look quite different in size when viewed from above.
For the best indication of a diamond’s face-up size, always look at its measurements as well as its carat weight.
Colour Grade
White diamonds are assigned a colour grade on a scale ranging from D to Z. This scale describes the degree of colour present in the diamond, with D representing the least colour and the grades progressing towards increasingly noticeable yellow or brown colour.
Limitations of Colour Grades
A colour grade tells you the overall degree of colour in a diamond, but it doesn't necessarily describe the exact hue or undertone. While yellow is the colour most we most commonly expect a diamond to have on D–Z grading scale, diamonds can also have brown, grey or other subtle colour undertones. These affect how a diamond looks and, in turn, its desirability and value.
For example, in the image below, we have two diamonds that are both graded J colour. However, the diamond on the left has a brown tint, while the diamond on the right has a more typical yellow tint. Despite having the same colour grade, the brown-tinted diamond appears darker and more noticeable in colour. This makes it look lower in colour than the other diamond and also affects its value.
Clarity Grade
Diamonds can contain small natural features known as inclusions, which are formed as the diamond develops. The clarity grade describes the extent of these internal characteristics, taking into account factors such as their size, number, position, nature and visibility.
The GIA clarity scale ranges from Flawless (FL), meaning no inclusions or blemishes are visible under 10× magnification, through to Included 3 (I3), where inclusions are usually obvious and can have a significant effect on the diamond's appearance.
Limitations of Clarity Grades
There can be considerable variation between diamonds with the same clarity grade, particularly in the VS2 and lower categories. This is because inclusions vary in their size, colour, position and visibility. It's therefore important to look at the individual diamond rather than relying on the clarity grade alone.
For example, two diamonds could both be graded SI2. One might have several small, dispersed inclusions towards the edge of the stone, while the other could have a large, dark inclusion near the centre. On paper, they have the same clarity grade, but in reality they could look very different.
For diamonds over 1 carat, a GIA report will generally include an inclusion plotting diagram. This is a map showing the approximate position and type of inclusions identified by the grader. It can be useful for understanding the diamond's characteristics, but it is not a substitute for looking at the stone itself.
In the examples below, the first diamond has a plotting diagram that might initially look quite concerning. However, when we look at the actual diamond, the inclusions are very difficult to see. The second diamond appears to have a more favourable plotting diagram, but the image reveals a dark inclusion in the centre that is visible without magnification.
This is a good example of why the plotting diagram and clarity grade should be considered alongside images or video of the actual diamond.
Cut, Polish, Symmetry and the Proportions Diagram
A GIA report records the diamond's polish and symmetry grades and, for round brilliant diamonds, an overall cut grade. GIA does not currently provide an overall cut grade for fancy shapes, such as oval, pear, emerald or cushion cuts.
The grades range from Excellent, followed by Very Good, Good, Fair and Poor. The overall cut grade for a round brilliant diamond considers factors that influence its light performance and appearance. The report also includes a proportions diagram, showing measurements and angles that help describe how the diamond has been cut.
Limitations of the Cut Grade
The cut grade is an excellent starting point, and for round brilliant diamonds we generally recommend looking for a Triple Excellent grade, meaning Excellent cut, polish and symmetry.
Fancy shapes are more complicated when it comes to assessing cut, and deserve a separate article of their own, so we'll focus on round brilliant diamonds here.
Even within the Excellent cut category, there is a fairly broad range of proportions. The way these proportions interact with one another plays an important role in how well a diamond handles light and, ultimately, how much it sparkles.
For example, the two diamonds pictured below are the same carat weight, colour grade and clarity grade, and both have an Excellent cut grade. However, the proportions of the diamond on the right are more balanced than those of the diamond on the left. This difference can affect their light performance, meaning the diamond on the right would be brighter and more lively diamond.
This is why we look beyond the overall cut grade and consider the individual proportions, alongside additional imagery and analysis, such as ASET images and Hearts and Arrows charts.
Fluorescence
Fluorescence describes how a diamond reacts when exposed to ultraviolet (UV) light. Some diamonds emit a visible glow, most commonly blue, and a GIA report records the strength of this reaction.
Limitations of Fluorescence
In the vast majority of cases, fluorescence has no perceptible effect on a diamond's appearance. However, in a very small number of diamonds, particularly those with very strong fluorescence, it can contribute to a hazy or milky appearance. This is generally associated with internal structural features that scatter light, rather than fluorescence alone.
Other characteristics can also affect a diamonds lustre. For example, poor polish can affect the quality of its surface finish, while internal graining can sometimes affect its transparency. These characteristics are not always obvious from the overall grades on a report, so it's important to assess the actual diamond.
So, How Much Can a Diamond Grading Report Really Tell You?
A diamond grading report is an invaluable starting point. It provides an independent assessment of the diamond's characteristics and allows you to compare stones using a consistent set of criteria.
However, a report can only tell you so much. It cannot fully communicate how a diamond looks in person, how lively it appears, or how its individual characteristics interact. Two diamonds with very similar grades can look surprisingly different when viewed side by side.
This is why we believe it's important to look beyond the report itself, considering the proportions, images and additional analysis of the actual diamond. Ultimately, the report helps us understand what we're looking at, but it's the individual stone that matters.
