The labs, ranked by how strict they are
| Lab | Reputation | Where you see it |
|---|---|---|
| GIA | The reference standard, strictest | Most natural stones of value |
| AGS | Comparable to GIA, strong on cut | Less common now |
| IGI | Solid, slightly looser on natural | Dominant for lab grown |
| HRD | Well regarded, European | Common in Europe |
| EGL | Historically generous | Trades at a discount for it |
| In-house | Not independent | Treat as a sales document |
Why the lab changes the price
Grading is not fully mechanical. A stone whose inclusions put it right on the boundary between VS2 and SI1 can be defensibly called either. Multiply that judgement across colour as well and two labs can honestly differ by a grade or two on the same stone.
The trade knows exactly which labs lean generous and prices accordingly. So a stone certified as G VS1 by a loose lab may trade like an H VS2, and if you paid a G VS1 price you overpaid. This is why comparing two listings by their stated grades alone is a mistake.
How to check a certificate is real
Every major lab has a free report lookup on its website. Type in the report number and the grades should come back matching the paper in your hand. If they do not, or the number returns nothing, you have your answer.
Then match the certificate to the actual stone. The report lists measurements to a hundredth of a millimetre and carat weight to a hundredth. Stones of real value are also laser inscribed on the girdle with the report number, readable under magnification. A certificate that does not demonstrably belong to the stone in front of you is just a piece of paper.
When certification is not worth it
Under about half a carat the cost of grading eats a real percentage of the stone's value, and most small stones sold in jewellery are uncertified for that reason. That is normal and not a red flag.
Above half a carat, and certainly at a carat and above, buying without an independent report means accepting the seller's own grading on something worth thousands. If you do it, expect and negotiate a discount for the uncertainty.
Not sure what you are looking at?
Photograph the stone and the app gives you a visual read on the 4Cs, an estimated value range, and what to check next. Free on iOS and Android.
Frequently asked questions
Which lab is strictest?
Does a certificate prove a diamond is real?
Why would the same stone get different grades?
Is an uncertified diamond a bad buy?
Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.