A grading report can look like the part of the decision that settles everything. It is more useful read as what it is: a precise record of a defined set of observations about one stone.
This is not a case for trusting documents less. It is a case for reading them for what they say.
What a report is describing
A report describes one stone as a laboratory examined it, at the time it examined it. Read it as a record of observations, not as a valuation or a price, and not as a prediction of how the stone will look in the room where you see it. Such a document is sometimes called a certificate; whatever it is called, the useful questions are what exactly it states, and about which stone.
Two reports can each be careful and still not line up, because you cannot assume that two laboratories grade the same things in the same way. So the first question about any document is who issued it, and the second is what that issuer grades for this kind of stone.
Measurements describe the outline
Where a report gives measurements in millimetres, length and width — or the diameter, for a round stone — describe the outline you will see from above. Depth is measured in profile, and does not appear in the face-up view at all.
Carat weight is a separate matter. Carat measures weight, not size: one metric carat is 200 milligrams. In a round brilliant cut too deep, or with too thick a girdle, extra weight can sit in or below the girdle, where it adds nothing to the outline you see from above. Two stones of the same weight can therefore look different in size, and the face-up measurements deserve as much attention as the carat figure.
The measurements are dimensions, not a measure of how large a stone looks on a hand. Connecting the two is something you do by looking at the stone.
Do not assume a cut grade for every shape
This entry deserves care, because a misreading changes what you think you are comparing.
An overall cut grade cannot be assumed for every shape. On GIA reports, for example, the standard round brilliant is currently the only shape that receives an overall cut grade; fancy shapes — every outline other than round — receive none. So where the report for an oval or a pear carries no cut grade, that is not by itself a mark against the stone. Ask what the issuing laboratory grades for that shape.
A fancy shape with no cut grade and a round with a high one are therefore not two points on the same scale. If you are comparing an oval with a round, the documents will not make the comparison for you; the straightforward way to make it is to look at both under the same light.
Where a cut grade is given, it concerns how the stone’s proportions and facets affect the way it interacts with light: brightness, the light the diamond returns; fire, light split into the colours of the spectrum; and scintillation, the sparkle and the pattern of light and dark areas as the stone moves.
Polish and symmetry are separate entries
Polish concerns the finish of the stone’s surfaces. Symmetry concerns how exactly its shape and facets are made and placed. GIA grades both separately, and a report for a fancy shape may carry polish and symmetry grades even though it carries no overall cut grade — which is one reason they can be mistaken for one.
They are worth reading, and they are not a substitute. A stone can be finely finished and precisely made and still have proportions you do not prefer.
A fluorescence entry is a description, not a verdict
Where a report records fluorescence, it describes how strongly the stone reacts to ultraviolet light. Whether a report records it at all depends on the report. On its own, the entry is a description, not a verdict on the stone.
GIA research published in 1997 studied how blue fluorescence affects the appearance of diamonds. In its observations, strongly blue fluorescent round brilliants were judged to have a better colour appearance face-up, with no clear trend when the stones were viewed face-down. That is a finding about appearance under the study’s viewing conditions, not a change to any recorded grade.
If fluorescence matters to you, look at the stone in daylight, then in ordinary indoor light, and see what you can actually see.
What changes once the stone is set
Some of what a report records cannot be checked once a stone is set, and it is worth knowing which.
A stone in a setting cannot be weighed on its own. Prongs and bezel rims cover parts of the girdle and crown, and metal cannot be seen through, so an inclusion or a blemish near the edge that lies under the metal is out of view. The setting also hides the stone’s profile, and the colour of the metal around it can influence how the stone’s own colour looks.
None of this makes a finished piece a lesser piece. It means that a report for a set stone describes an examination that may have taken place before it was set, and that the areas under the metal cannot be inspected without unsetting the stone.
Two reasonable questions follow. Was this stone graded before it was set, and how is the document matched to it now — for example by an inscription, a plotted diagram or a combination of measurements? And what does the setting cover?
Matching the document to the stone in front of you
Read the identifying details — shape, measurements and weight — and check that they correspond to the stone being shown to you. If the document refers to an inscription or another identifier, ask how it is checked.
Then set the document aside and look. A report describes; it does not replace looking. Who issued a document, what it covers and which stone it belongs to, and the characteristics of a specific diamond, are confirmed directly with the company selling it before a purchase.
The short version
Four questions to take with you.
- Who issued this document, and what does it grade for this shape?
- Which of its entries can be checked by looking at the stone, and which cannot?
- Was the stone graded before it was set, and how is the document matched to it now?
- What is not on the report at all?
A document that answers those is doing its job. It was never the whole decision.