What Actually Happens at a Grading Laboratory

Lab Grown Diamonds

Knowing how a report is produced tells you exactly how much weight each line on it can carry.

Written by the Mapple Gems workshop, Surat · Updated 1 October 2026

Short answer

A stone arrives anonymously, is cleaned, weighed and measured by machine, then assessed by several graders independently — colour face-down against master stones, clarity at 10× magnification. Proportions are measured optically; cut is calculated from those measurements and only on round brilliants. Finally the report number is laser-inscribed on the girdle, which is the only physical link between the paper and the stone. Everything a report can and cannot tell you follows from that process.

Grading reports are treated as verdicts. They are closer to a standardised measurement record, and knowing how each figure is produced tells you which ones to trust absolutely and which ones need a second look.

Colour and clarity are human judgements made by several people under conditions designed to reveal differences you will never encounter — while the measurements are machine output. That is the single most useful thing to understand about a report. Colour is assessed with the stone face-down in a white tray beside master stones of known grade, under standardised lighting — a position that exposes body colour maximally and one your ring will never be in. Clarity is assessed at exactly 10× magnification by graders who are looking for inclusions on purpose. Both are then reconciled between independent assessments, which is why grading is consistent rather than arbitrary — but it is still a judgement about the stone in laboratory conditions. The measurements line is different in kind: diameter, depth percentage, table percentage and girdle are measured optically, to hundredths of a millimetre, with no judgement involved at all. So when a report and your eyes disagree, trust the measurements and re-check the grades — and remember that a grade earned face-down against white is not a prediction of how the stone looks face-up in warm gold.

The sequence

Stage	How it is done
Intake	Stone separated from its owner’s identity
Cleaning and weighing	Precision scale, to the hundredth of a carat
Measurement	Optical scanning — machine, no judgement
Colour	Face-down vs masters, several graders
Clarity	10× magnification; the plot is drawn here
Cut	Calculated from proportions — rounds only
Origin screening	Specialist instruments identify lab grown
Inscription	Report number lasered on the girdle

Why the inscription is the whole system

A report describes a stone, not a ring — and on its own, a piece of paper describing “a 1.02 ct round, G, VS1” describes thousands of stones.

The laser inscription on the girdle, matching the report number, is the only thing tying that document to the stone in front of you. It is why laboratories inscribe lab grown stones as a matter of course, and why the two checks that actually matter are: read the inscription under magnification, and verify the number on the laboratory’s own website rather than trusting a supplied PDF. Both take under a minute.

It also means a stone that has been recut or damaged and repolished may have lost its inscription — which is worth knowing when buying anything second-hand.

Why cut is graded only on rounds

Because a cut grade is calculated from proportions against a reference model of how light should behave, and that model only exists where the geometry is standardised. Round brilliants are; ovals, pears, cushions and the rest are not.

So on every fancy shape there is no cut grade at all, and the responsibility transfers to you. That is why we keep saying the same thing: on anything but a round, rotating video in ordinary light is the grading. It is the only way to see a bow-tie, an uneven outline, or a dead centre — none of which appear anywhere on a report.

Which laboratory, and why it now matters

Reports are not interchangeable, and one change is recent enough that many buyers have not met it.

GIA moved to Premium and Standard descriptors for lab grown stones on 1 October 2025, while IGI and GCAL continue to issue full 4Cs. So a GIA lab grown report and an IGI one cannot be compared cell by cell — they carry different levels of detail. If you want the full grading breakdown on a lab grown stone, that currently means IGI or GCAL.

Beyond the major laboratories, grading standards vary. Comparing a well-known lab’s report against a lesser-known one is comparing two different scales, not two stones — which is a real source of apparent bargains that are not.

What a report will never tell you

Anything about the ring — metal, gram weight, shank thickness, setting quality, accent stones. All of that is the piece, not the stone.
Whether it is eye-clean. Clarity is graded at 10×; you wear it unmagnified. Open the plot diagram — position beats grade.
How it looks in your setting. Colour is graded against white; warm metal changes what you see.
Cut quality on any fancy shape.
What it is worth. A report is not a valuation and a valuation is not a market price.

And one thing no laboratory does at all

Nobody grades moissanite. No major laboratory issues colour, clarity or cut grades for it on the diamond scales, so the “D–E–F” and “VVS” on a moissanite listing are seller assessments — not comparable between sellers, and not verifiable against anything.

That is not a criticism of the material; it is a structural fact about it, and it has one consequence worth carrying: with no independent anchor, a higher price is not evidence of a better stone. What replaces the report is measured millimetres and rotating video in ordinary light.

Keep reading

Frequently asked questions

How is a diamond actually graded?

The stone arrives anonymously, is cleaned, weighed and optically measured by machine, then colour is judged face-down against master stones and clarity at 10× magnification by several graders independently. Cut is calculated from the proportions, and only on round brilliants. The report number is then laser-inscribed on the girdle.

Why is colour graded face-down?

Because that position exposes body colour maximally against a white tray beside master stones — conditions designed to reveal small differences consistently. It is not how you wear the stone, which is why a grade earned face-down is not a prediction of how it looks face-up in warm gold.

Why do fancy shapes have no cut grade?

Because a cut grade is calculated from proportions against a reference model of light behaviour, and that model only exists where the geometry is standardised — which round brilliants are and other shapes are not. On any fancy shape, rotating video in ordinary light is the grading.

Is a report proof that a stone is what it says?

Only in combination with the girdle inscription, which is the sole physical link between the paper and the stone. Read the inscription under magnification and verify the number on the laboratory’s own website rather than trusting a supplied PDF. Both take under a minute.

Read the inscription, then verify the number.

Under a minute, and it is the difference between a report about your stone and a report about a stone.

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