A diamond grading lab examines a loose diamond and issues a report describing its identity, its 4Cs and any treatments it detects. The best-known labs are GIA, IGI and HRD Antwerp, and until the end of 2022 AGS Laboratories was also a major name. Labs use the same letters and terms, but they do not all apply them with the same strictness, so the name on the report matters when comparing stones.
Why independent labs exist
A seller has an obvious interest in describing a diamond generously. Independent labs grew up to give buyers and sellers a neutral description they could both rely on. A lab that does not buy or sell diamonds has no stake in the outcome of a sale, and its report travels with the stone from dealer to dealer.
Grading labs do not set prices or appraise value. They describe the stone. Turning that description into a price is the job of the market, and estimating a replacement value for insurance is the job of an appraiser. The guide to diamond certificates explains that difference in more detail.
GIA
The Gemological Institute of America was founded in 1931 by Robert M. Shipley as a school for jewelers. It is a nonprofit organization headquartered in Carlsbad, California, with laboratories in several countries. In the early 1950s GIA created the D-to-Z color scale and the Flawless-to-Included clarity scale described in the diamond 4Cs guide, and it began issuing grading reports in the 1950s.
GIA reports are the most widely used reference in the diamond trade. Its grading scales became the common language that other labs adopted, and many dealers treat a GIA grade as the benchmark when comparing stones from different labs. GIA also runs gemology education programs and publishes the research journal Gems & Gemology.
AGS Laboratories
The American Gem Society was founded in 1934, also by Shipley, as a trade association of jewelers. It opened its own lab, AGS Laboratories, in 1996. The AGS lab became known for a numeric 0-to-10 grading scale and for cut grades based on light performance, using tools such as ASET imaging.
In October 2022, AGS and GIA announced that the AGS lab's research staff, technology and Las Vegas facility would become part of GIA. AGS Laboratories stopped grading at the end of 2022, and starting in 2023 GIA offered an AGS Ideal report as an add-on for eligible diamonds. The American Gem Society continues as a separate trade organization. The AGS diamond report page covers the scale and its history.
IGI
The International Gemological Institute was founded in Antwerp in 1975. It operates laboratories in many diamond trading and cutting centers, and it grades a large share of the diamonds sold through retail chains. IGI became one of the most common labs for lab-grown diamonds as that market grew in the late 2010s and 2020s. Its reports use the same D-to-Z color and FL-to-I3 clarity terms.
HRD Antwerp
HRD Antwerp is the grading arm of the Antwerp World Diamond Centre. Its name comes from Hoge Raad voor Diamant, Dutch for Diamond High Council, and its lab dates to the 1970s. HRD grades according to international nomenclature used in Europe, which calls the top clarity grade "loupe clean." Its reports are common for stones traded through Antwerp, the city whose history is covered on the Antwerp diamond bourses page.
Other labs
Many other labs grade diamonds. Some are regional, some offer a narrow set of services, and some focus on colored gemstones. Labs such as the Swiss Gemmological Institute SSEF and the Gubelin Gem Lab are best known for colored stones and origin reports, though they also test diamonds. Some retailers and manufacturers also run in-house grading, which is not independent.
When you meet an unfamiliar lab name on a report, ask the seller how that lab's grades compare with GIA's and look the lab up for its history, ownership and methods.
Why grades can differ between labs
Diamond grading involves judgment. Color is graded by comparing a stone with master stones under controlled light, and clarity is graded by deciding how visible inclusions are at 10x. Two skilled graders can disagree by a grade on a borderline stone, which is why large labs have several graders look at each diamond.
Differences between labs go beyond single borderline calls. Labs use their own master stone sets, train graders to their own standards and draw their grade boundaries in slightly different places. Over time, the trade has observed that some labs grade more leniently than others. Dealers account for this by pricing stones partly by lab. A diamond graded G by one lab may trade at a different price from a diamond graded G by another, because buyers do not assume the two letters describe the same color.
For a buyer, the practical lesson is to compare stones graded by the same lab, or to adjust expectations when comparing across labs. A lower price on a stone from a lab with looser standards may simply reflect a lower true grade.
What a grading report includes
A full diamond grading report usually records:
- Report number and date, which allow online verification.
- Shape and cutting style, such as round brilliant or emerald cut.
- Measurements in millimeters and carat weight to two decimal places.
- Color grade, clarity grade and, for round brilliants at most labs, a cut grade.
- Polish, symmetry and fluorescence.
- A proportions diagram with table, depth, angles, girdle and culet.
- A clarity plot showing the main inclusions and blemishes.
- Comments, such as notes on treatments or a laser inscription on the girdle.
The report also states whether the diamond is natural or laboratory-grown and notes any detected diamond treatments, such as laser drilling or HPHT processing. Some labs decline to grade fracture-filled stones.
Identification as well as grading
Major labs do more than assign grades. They screen every stone to confirm that it is diamond, not a simulant such as moissanite or cubic zirconia, and they test whether it is natural or lab-grown. This work uses spectroscopy, fluorescence imaging and other instruments that go well beyond the tools a jeweler has at the counter. The how to tell if a diamond is real guide explains why lab testing is the reliable answer.
Frequently asked questions
Which diamond grading lab is best?
GIA is the most widely used reference in the trade, and its grades are often treated as the benchmark. Other labs are reputable, but buyers should compare stones graded by the same lab.
Do grading labs appraise diamonds?
No. A grading report describes quality and identity but gives no monetary value. Appraisals for insurance come from independent appraisers.
Does AGS still grade diamonds?
AGS Laboratories stopped grading at the end of 2022 when its research and technology moved to GIA. GIA now offers an AGS Ideal report as an add-on for eligible diamonds.
Can two labs give the same diamond different grades?
Yes. Grading involves judgment, and labs draw their boundaries in slightly different places, so a stone can receive different color or clarity grades at different labs.