A grading report is a laboratory’s opinion, recorded on a date, about one stone. Read properly it will tell you almost everything. Read badly it will sell you a diamond you do not want.
Who issued it, and does that matter?
It matters a great deal. Laboratories are not equally strict, and a grade is only worth the house that wrote it.
- GIA — the Gemological Institute of America wrote the 4Cs in the middle of the last century and remains the reference standard for natural diamonds. If a stone is significant, this is the report you want.
- IGI — widely used, and now the dominant laboratory for lab-grown stones. Reliable, and the sensible choice for a grown diamond.
- HRD — Antwerp’s laboratory, respected across Europe.
- In-house or unnamed reports — treat with real caution. A report written by the seller is a description, not an independent opinion.
One practical consequence: never compare a price on a GIA stone with a price on an in-house-certified stone and conclude the second is better value. You are comparing two different claims.
The header: report number and date
Every report carries a number. On GIA reports the number is frequently laser-inscribed on the girdle of the stone in letters about a hundredth of a millimetre tall — ask to see it under the microscope, because that inscription is what ties the paper in your hand to the diamond on the table. Any jeweller on Hatton Garden will show you without being asked twice; if someone hesitates, leave.
Check the date too. Reports do not expire, but a report from a decade ago describes a stone that may since have been re-polished, damaged or re-set.
Shape, measurements and weight
Measurements appear as three numbers: maximum diameter, minimum diameter, depth. They are worth more attention than the carat weight. A 1.00ct round brilliant should measure roughly 6.4 to 6.5 mm across; if it measures 6.1 mm, weight is hiding in the pavilion and the stone will look small for its price.
On fancy shapes, divide length by width to get the ratio. An oval at 1.35 is classic; at 1.55 it is dramatic and will look longer on the finger. Neither is better — but the number tells you what you are getting before you see it.
The four grades
Colour, clarity, cut and carat appear together. Two notes people miss. First, cut grade only exists for round brilliants on most reports — fancy shapes show polish and symmetry instead, which is why your own eyes matter more there. Second, polish and symmetry are separate lines from cut; Excellent cut with Good symmetry is common and perfectly acceptable, while Fair symmetry on a round brilliant is a reason to look at another stone.
The proportions diagram
The little cross-section on the right of the report is the most under-read part of the page. Four numbers carry most of the meaning:
| Measure | Comfortable range | What goes wrong outside it |
|---|---|---|
| Table % | 53 – 58 | Large tables look flat; small tables lose brightness |
| Total depth % | 59 – 62.5 | Deep stones face up small; shallow stones window |
| Crown angle | 34 – 35° | Flat crowns lose fire |
| Pavilion angle | 40.6 – 41° | The single biggest driver of light return |
Also on that diagram: girdle thickness and culet. A girdle described as Extremely Thin is a chip waiting to happen in a claw setting; Very Thick is weight you paid for and cannot see. Medium to Slightly Thick is where you want to be.
The clarity plot
The plot is a map of the stone’s inclusions: red marks are internal, green marks are surface blemishes. Read it for position, not quantity. One crystal directly under the table is worse than four small feathers out by the girdle, because the girdle disappears under the setting and the table does not.
Watch for the word feather sitting on the girdle edge. Feathers are fractures; a feather breaking the surface at the girdle is the one clarity feature that has any bearing on durability, and it is worth asking a setter about before you commit.
By appointment · Hatton Garden
Bring the report, we will read it with you
If you are considering a stone from anywhere on Hatton Garden — or online — bring or send the report number. We will pull up the laboratory record and talk you through the proportions and the plot before you spend anything.
Fluorescence
Roughly a third of diamonds glow, usually blue, under ultraviolet light. Reports grade it None, Faint, Medium, Strong or Very Strong. The trade discounts strong fluorescence, which means it is frequently a bargain: in a lower colour grade, blue fluorescence can actually make a stone face up whiter. The risk is a hazy or oily look in direct sunlight, which affects a minority of strongly fluorescent stones. So: do not rule it out, but do take the stone to a window.
Comments — the line nobody reads
At the bottom of every report is a comments box, and it occasionally contains the most important sentence on the page. Things that appear there include additional clouds not shown on the plot, internal graining, or a note that clarity is based on clouds not shown. A stone graded SI1 whose clarity is driven by an unplotted cloud can look milky, and the report has told you so in the smallest type on the page.
What a report will never tell you
It will not tell you whether the stone is beautiful. Two diamonds with identical reports can perform very differently, because contrast, the pattern of the facets and the way light moves when the hand moves are not gradable. It will not tell you about the setting, the workmanship, or whether the ring will still be sound in thirty years.
Which is the argument for buying on a street where the stones are in front of you. Hatton Garden exists because looking is not optional.
By appointment · Hatton Garden
See two identical reports, two different stones
It is the most persuasive demonstration in the trade, and it takes five minutes. Book a viewing and we will show you what a certificate cannot print.
Written on Hatton Garden. Grading terminology follows GIA definitions; prices are indicative at the time of writing and move with the market. Nothing here is a valuation.
