Lab Grown Diamonds
Two Stones, Identical Grades, Different Prices
Same carat, same colour, same clarity, same cut grade β and one is meaningfully better. Here is where to look.
Written by the Mapple Gems workshop, Surat Β· Updated 21 September 2026
Short answer
The four grades are a summary, not the whole report. When two stones look identical on paper, the differences sit in the lines nobody reads: the measurements (is it cut to size or spread shallow), depth and table percentages, polish and symmetry as separate grades, and the plot diagram β because where an inclusion sits matters more than its grade. Check those five, and if the stones still match, take the cheaper one.
This is the moment most lab grown buying actually gets decided, and it is where the paperwork stops helping unless you know which lines to open.
The five lines to compare
| Line on the report | What good looks like (round) |
|---|---|
| Measurements | 6.5 mm at 1.00 ct; larger = spread shallow |
| Depth % | 59 β 62.5% |
| Table % | 53 β 58% |
| Polish & symmetry | Both Excellent or Very Good β two separate grades |
| Girdle | Thin to slightly thick; not extremely thin or very thick |
| Plot diagram | Inclusions away from directly under the table |
The plot diagram is printed on every report, almost nobody opens it, and it frequently matters more than the clarity grade itself. Clarity is graded on the size, number and nature of inclusions β but the grade does not tell you where they are, and position is what you actually see. An inclusion sitting directly beneath the table is in the clearest line of sight through the stone; the identical inclusion tucked near the girdle, where a prong will cover it, is invisible for the life of the ring. So a well-positioned SI1 routinely outperforms a badly positioned VS2 face-up β and costs less. This is the single most useful thing a buyer can do with a report and it takes about a minute: open the plot on both stones, look at where the marks cluster, and prefer the one with a clear centre. It is also why step cuts need one grade higher: an emerald or Asscher has a large open table with nothing to hide behind.
Measurements: the fastest tell
Compare the millimetre figures against the standard for the weight. A 1.00Β ct round should measure about 6.5Β mm.
Measuring larger is not a bonus. It means the stone was cut shallow to spread the weight across a wider face β more surface, less light returned, and often a visible dullness in the centre. Measuring smaller means weight is hidden in an over-thick girdle or excess depth you paid for and cannot see.
Two stones at the same stated weight with different measurements are not the same purchase, and this line appears on every report.
Polish and symmetry are two grades, not one
They get quoted together and describe different things. Polish is the surface finish of each facet; symmetry is whether facets meet where they should. A stone can be Excellent on one and Good on the other, and listings often quote only the better figure.
Ask for both. Symmetry is the one that affects light return more, particularly on fancy shapes where there is no cut grade to fall back on.
Things that are not real differences
- Growth method. CVD and HPHT produce different inclusion types, but a finished, graded stone is graded on what it is β not on how it was made.
- A colour grade above what the metal shows. In yellow or rose gold, GβH reads white; paying for D is paying for a difference the setting removes.
- A clarity grade above eye-clean below about a carat. Nobody views a ring at 10Γ.
- Fluorescence in most cases β it usually attracts a discount rather than indicating a problem, so it can be a genuine saving.
Which laboratory issued it
Worth checking, because the reports are no longer directly comparable. GIA moved to Premium / Standard descriptors for lab grown on 1 October 2025, while IGI and GCAL still issue full 4Cs. So a GIA-graded lab grown stone will not show the same detail as an IGI one, and you cannot line the two up cell by cell.
Grading standards also vary between smaller laboratories. Comparing an IGI report against a lesser-known labβs report is comparing two different scales, not two stones.
Before you decide
- Ask for rotating video of both, in ordinary indoor light rather than a lightbox β a lightbox flatters every stone equally and hides the differences you are trying to find.
- Check the girdle inscription against the report number. That inscription is the only thing tying a report to a specific stone.
- Verify the report on the laboratoryβs own site using the number, rather than trusting a supplied PDF.
- If they still match, buy the cheaper one β and ask the seller why it is cheaper. A good answer exists in most cases, and the question itself is informative.
Keep reading
Frequently asked questions
Why do two diamonds with the same grades cost different amounts?
Because the four grades are a summary. The differences sit in the measurements, depth and table percentages, polish and symmetry as separate grades, the girdle thickness, and the plot diagram showing where inclusions actually are. Check those five before assuming two stones are equivalent.
Does inclusion position matter more than clarity grade?
Frequently, yes. An inclusion directly beneath the table sits in the clearest line of sight, while the identical inclusion near the girdle is covered by a prong and invisible for the life of the ring β so a well-positioned SI1 routinely outperforms a badly positioned VS2 face-up, and costs less.
What do the measurements on a report tell me?
Whether the weight was cut into size or spread. A 1.00Β ct round should measure about 6.5Β mm; larger means it was cut shallow, with more surface and less light returned, and smaller means weight is hidden in an over-thick girdle or excess depth you cannot see.
Can I compare a GIA report against an IGI report?
Not cell by cell for lab grown stones. GIA moved to Premium and Standard descriptors for lab grown on 1 October 2025 while IGI and GCAL still issue full 4Cs, so the two reports carry different levels of detail. Grading standards also vary between smaller laboratories.
Send us both report numbers.
We will open the plots, compare the measurements against standard, and tell you which stone is the better buy β including if it is not ours.
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