Published on18/08/2026
A grading report is not a certificate of quality. It is a description: an independent lab measured the stone and wrote down what they found. That sounds obvious, but misunderstanding it is the root of nearly every mistake people make buying diamonds.
GIA, the Gemological Institute of America, created the 4C scale the whole industry now uses. It is also regarded as the strictest and most consistent lab in the world. Send the same stone to GIA twice, years apart, and you will very likely get the same result both times.
IGI, the International Gemological Institute, is the second largest by volume and grades more generously overall. In practice, an IGI stone marked E VS1 is typically the equivalent of a GIA stone marked F VS2, and the gap on Colour sometimes reaches two grades. This does NOT mean IGI reports are fake or that IGI is dishonest. It means the two labs use slightly different rulers.
The consequence for your wallet is very concrete. If you are looking at two stones — one with a GIA report reading F VS2 at 90 million dong, one with an IGI report reading E VS1 at 75 million — the instinct is to take the second, because the specs are better and the price is lower. But if that IGI stone is really an F VS2 on the GIA ruler, you are comparing two EQUIVALENT stones, and the 15 million gap reflects the market correctly rather than a bargain.
The practical rule: only compare prices between stones certified by the SAME lab. Cross-comparing between labs means comparing two different units of measurement.
Beyond GIA and IGI there are many other labs. Some have a solid reputation within a narrow scope. Some grade so loosely that the report carries almost no information. The safe principle: if you have never heard of the lab, treat the stone as uncertified and price it on what you can see with your own eyes.
For stones under 0.3ct, certification costs too large a share of the stone value, so nearly all melee is sold without individual reports. That is entirely normal industry practice, not a red flag. From 0.3ct up a report is advisable, and from 0.5ct up it is essential.
The single most important step is the one most often skipped: CHECK THE INSCRIPTION. Most certified stones carry a microscopic laser-inscribed number on the girdle, and that number must match the one printed on the report. Ask for a 10x loupe at the counter and read it yourself. A genuine report paired with a different stone is the most common switch there is, and this step is the only place it gets caught.
Next, look at the PLOTTING DIAGRAM printed on the report. It maps where each inclusion sits inside the stone, and that is far more useful information than the Clarity grade itself. An SI1 with its inclusion near the girdle, where a prong will cover it, is in practice a better stone than a VS2 with its inclusion dead centre under the table. The grade does not tell you that; the diagram does.
Finally, verify the report is current and belongs to this stone by entering the report number on the lab own website. Both GIA and IGI provide free public lookup tools. It takes thirty seconds, and it completely eliminates the possibility that the paper in your hand is a forgery.