Reading a Diamond Grading Report Line by Line – Mapple Gems

Lab Grown Diamonds

Reading a Grading Report Line by Line

Most people read three lines out of twenty. Here is what the other seventeen tell you.

Written by the Mapple Gems workshop, Surat · Updated 14 September 2026

Short answer

Everyone reads carat, colour and clarity. The lines that actually change a decision are measurements (check them against the stone), depth and table percentages (whether it is cut for beauty or for weight), girdle thickness (chipping risk at one extreme, hidden weight at the other), polish and symmetry (should both be Excellent or Very Good), and the plot diagram (where the inclusions sit — position beats grade). Plus the inscription number, which is the only thing tying the report to your stone.

A grading report is a dense technical document that most buyers skim for three figures. The remaining fields are where the real information is, and none of them require expertise to use.

The header fields

Field	What to do with it
Report number	Look it up on the laboratory’s own site, typed directly
“Laboratory grown”	Must be stated explicitly — it always is on a genuine report
Growth method	CVD or HPHT — informational, not a quality ranking
Shape and cutting style	Check it matches what you are buying
Measurements (mm)	Check against the physical stone — the cheapest verification there is

The inscription number is the only line that connects the report to your stone. Everything else describes a stone with those characteristics. Reports are issued to loose stones before setting, so a genuine, verifiable report proves such a stone exists — not that it is the one in your ring. The link is the laser inscription on the girdle, a marking of the report number readable under 10× magnification. It is printed on the report and it takes a jeweller ten seconds to read against it. Ask the seller to confirm the stone is inscribed, and have it checked at handover or at your first service. This is not an accusation of anyone; it is simply the one step that ties the paperwork to the object, and it is the step routinely skipped.

The proportions section

This is where cut quality lives, and where a stone reveals whether it was cut for beauty or for weight retention.

Depth % — the stone’s height as a proportion of its width. Very high means weight hidden below where you cannot see it; very low means a spread, shallow stone that leaks light. On a round, roughly 59–62.5% is the comfortable band.
Table % — the flat top facet as a proportion of the width. An oversized table spreads the face and flattens the life; roughly 53–58% works well on a round.
Crown and pavilion angles — the two that actually determine light return. They matter most on rounds, where the cut grade already accounts for them.
Culet — “None” or “Pointed” on a modern brilliant. A stated open culet indicates an antique-style cut, which is a design choice rather than a fault.

For fancy shapes, remember these figures exist but no cut grade does — so read them alongside video rather than instead of it.

Girdle: the line almost nobody reads

The girdle is the stone’s outer edge, and the report states its thickness as a range — from Extremely Thin through to Extremely Thick.

Extremely Thin is a chipping risk, particularly on shapes with corners and during setting. Extremely Thick hides weight in a band of material you cannot see, so the stone measures smaller than its carat implies — you paid for weight doing no work.

Aim for the middle of the range, and treat either extreme as a reason to look at another stone. On a large purchase this single line can be worth more than a clarity grade.

Polish, symmetry and fluorescence

Polish is the surface finish and symmetry is the precision of the facet arrangement. Both are graded separately from cut, and both should be Excellent or Very Good — poor symmetry undermines an otherwise good cut grade, and it is one of the reasons two stones with identical cut grades can look different.

Fluorescence is a glow under ultraviolet light, invisible in ordinary lighting. It is more common in HPHT than CVD stones, and the mined trade’s pricing discount for it often carries over — which makes a fluorescent stone a genuine saving for anyone indifferent to how it behaves under a nightclub UV lamp.

The plot diagram

The single most useful part of the document, and the least opened.

It maps every inclusion onto a diagram of the stone. What you are looking for is position: an inclusion directly beneath the table sits in the clearest window with nothing to disguise it, while the same inclusion near the girdle is masked by the busy flashing at the edge or hidden by a prong.

That is why two SI1 stones at the same price can be completely different purchases. Ask for the plot before paying attention to the letter.

Which laboratory, and what that means now

IGI and GCAL issue full 4C reports for lab grown stones with cut, polish and symmetry detail. GIA moved to a Premium or Standard assessment for lab grown from October 2025, on the reasoning that production clusters near the top of both colour and clarity scales — which means a GIA report on a lab grown stone now carries less granular information than an IGI one.

A seller’s own printed card is a description, not a certification. “Certified” without a named laboratory and a lookup-able number means nothing checkable.

Keep reading

Frequently asked questions

Which lines on a grading report actually matter?

Measurements, which you can check against the physical stone; depth and table percentages, which show whether it was cut for beauty or weight; girdle thickness; polish and symmetry, which should both be Excellent or Very Good; the plot diagram showing where inclusions sit; and the inscription number, which ties the report to your stone.

What depth and table percentages should I look for?

On a round brilliant, roughly 59–62.5% depth and 53–58% table are comfortable. Very high depth hides weight below where you cannot see it; very low depth means a spread stone that leaks light. An oversized table spreads the face and flattens the stone’s life.

Does fluorescence matter on a lab grown diamond?

Only under ultraviolet light, where it appears as a glow — it is invisible in ordinary lighting. It is more common in HPHT than CVD stones, and the mined trade’s pricing discount for it often carries over, which makes a fluorescent stone a genuine saving for anyone indifferent to how it behaves under UV.

Why does the girdle thickness matter?

Extremely Thin is a chipping risk, especially on cornered shapes and during setting. Extremely Thick hides weight in a band you cannot see, so the stone measures smaller than its carat implies. Aim for the middle of the stated range — on a large purchase this one line can matter more than a clarity grade.

Ask for the plot and the inscription.

Both are already on the report. One tells you what the grade cannot; the other ties the whole document to your stone.

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