How to read a diamond grading report in ninety seconds
The four lines on a diamond grading report that a buyer should check at the counter with a loupe, before the colour and clarity grades.
Most buyers read a diamond grading report from the top of the grades down: carat, colour, clarity, cut. Those are the lines the price is built on, and they are the lines the seller has already read to you. At a counter or a trade stand, with a loupe in one hand and the report in the other, the more useful question is not what the grades say but whether this report belongs to this stone. That takes about ninety seconds if you know where to look.
A report describes a diamond on the day it was examined. It does not travel with the stone in any physical sense, and reports are separated from their stones, swapped and copied more often than the trade likes to admit. Four lines tie the paper to the object. Check them first, then read the grades.
The report number and the girdle inscription
Most major laboratories laser-inscribe the report number on the girdle, the thin outer edge of the stone. Under a 10x loupe, tilt the diamond until the girdle catches the light and read the digits. They should match the report exactly. If there is no inscription, that is not in itself a problem, since older and smaller stones often lack one, but it means the remaining three lines carry more weight. Most laboratories also offer an online check of the report number, which confirms that the report exists and returns the grades on file. It does not confirm that the stone in front of you is the one that was graded.
Measurements and carat weight
The report gives three measurements in millimetres, usually minimum diameter, maximum diameter and depth for a round, or length, width and depth for a fancy shape. These are recorded to the hundredth of a millimetre. A pocket gauge or a stand's calliper will not match to that precision, but it should come within a few hundredths. Carat weight is recorded to the hundredth of a carat, and a calibrated carat scale should agree with it exactly for an unmounted stone. A stone that weighs noticeably light is either not the stone on the report or has been recut since it was graded, and a recut stone needs a new report.
The plotting diagram
Reports for larger stones include a plot: a drawing of the crown and pavilion with the main inclusions marked in red and green symbols, keyed at the side. This is the diamond's fingerprint and the most reliable check you have at a counter. Pick the largest one or two plotted features, find their position on the drawing, then find them in the stone with the loupe. A feather near the girdle at four o'clock on the plot should be at four o'clock in the diamond when it is oriented the same way. If the obvious inclusion in the stone is nowhere on the plot, stop there.
The comments line
At the bottom of the grading section, often in small type, sits a comments field. It is where a laboratory records what does not fit elsewhere, and it is the line most often skipped. Read every word of it.
- Treatment notes, such as clarity enhancement, laser drilling or colour treatment, which change what a stone is worth and what must be disclosed to a customer.
- An identification of the diamond as laboratory-grown, sometimes repeated on the girdle inscription.
- Clouds or characteristics not shown on the plot, which can explain a clarity grade that looks generous at first glance.
- Notes on additional inscriptions, surface graining or other features that affect how the stone reads under light.
Fluorescence usually has its own line on the report. It is worth reading alongside the comments, since strong blue fluorescence can make a stone look hazy in daylight in a minority of cases, and many buyers want to see it before they commit.
Then read the grades
Once the four lines check out, the grades mean something. Note the report date, because grading standards and the stone itself can both change over years of wear. Note which laboratory issued it, because grades are not interchangeable across laboratories, and a colour grade from one is not always the same colour grade from another. Buyers heading to the Antwerp show in June 2027, where diamonds, rough and lab-grown lead the floor, will see reports from several laboratories in a single morning, and the habit of checking the number, the measurements, the plot and the comments before the grades is what keeps those reports comparable. Ninety seconds per stone is a small price for knowing that the paper and the diamond belong together.