Can a jeweller tell if a diamond is lab-grown?
A jeweller usually cannot tell if a diamond is lab-grown by eye or with a loupe; it takes a screening device, a gem laboratory, or the girdle inscription and its report. GIA inscribes "Laboratory-Grown" and its number on each lab-grown diamond it assesses, and an IGI lab-grown diamond report confirms the laboratory-grown origin. A thermal diamond tester reads both as diamond.
What can a jeweller see with a loupe?
Usually nothing that settles it. A lab-grown diamond is diamond, and GIA says colourless CVD-grown diamonds can look the same as natural ones to the unaided eye. When GIA screened 359 small yellow diamonds of 0.02 to 0.03 carat, 14 turned out to be HPHT lab-grown, and they could not be told apart by looking. GIA says that in some cases a trained gemmologist can recognise lab-grown diamonds with standard gem-testing equipment. HPHT diamonds often contain of the metal flux they grew in, which look black and opaque in transmitted light. Between crossed polarising filters, mined diamonds often show a crosshatched or mosaic strain pattern, while CVD diamonds tend to show banded strain. Under ultraviolet light, HPHT diamonds typically show a cross-shaped pattern, lab-grown diamonds often glow more strongly under short-wave than long-wave UV, and some keep glowing for a minute or more after the lamp goes off. None of these is proof on its own: GIA’s 2024 update on lab-grown identification says a conclusive identification typically requires a combination of features, and warns against relying on afterglow or any other single observation for colourless HPHT stones. A clean stone with none of these clues proves nothing either way. The two growth methods are explained in how lab gems are grown.
Do lab-grown diamonds test as real?
Yes. GIA says the chemical and physical properties of lab-grown diamonds correspond very closely to those of natural diamonds, so a lab-grown diamond passes a jeweller’s handheld thermal diamond tester just as a mined one does. That tester measures how well the stone conducts heat, which GIA describes as a trait measured in some instruments to distinguish diamond from many other simulants. It separates diamond from look-alikes, not mined diamond from lab-grown. GIA also notes that moissanite is sometimes mistaken for diamond because its thermal conductivity approaches diamond’s, so a thermal tester can be fooled by moissanite too (see does moissanite pass a diamond tester). A pass on a thermal tester therefore means "diamond, or possibly moissanite", never "mined". Telling lab-grown from mined takes a screening device built for that job, or a laboratory.
Which checks can tell lab-grown from mined?
| Check | Who can do it | Can it tell lab-grown from mined? |
|---|---|---|
| Eye or 10x loupe | Anyone | Rarely. A dark metallic inclusion can point to HPHT growth, but a clean stone tells you nothing |
| Thermal diamond tester | Any jeweller | No. It reads lab-grown and mined diamond alike, and can be fooled by moissanite |
| Magnet | Anyone | Sometimes, for HPHT stones with larger metallic inclusions. GIA says CVD diamonds do not have metallic inclusions, so no pull proves nothing |
| Microscope, polarising filters and UV lamp | A trained gemmologist | Sometimes: banded strain, a cross-shaped fluorescence pattern or a long afterglow are clues, not proof |
| Screening device, such as GIA’s iD100 or DiamondCheck | Jewellers and dealers who own one | Partly. Most screening tools give a "pass" or "refer" result for colourless to near-colourless stones: "pass" indicates natural; "refer" means natural or lab-grown, so the stone goes to a laboratory |
| Gem laboratory: deep-UV imaging, photoluminescence and | GIA, IGI and other gem laboratories | Yes. The laboratory combines several features to reach a conclusion |
| Girdle inscription and report lookup | A jeweller with a loupe or microscope, then you, online | Yes, if the inscription matches a real report on the laboratory’s own website |
What instruments do laboratories use?
Many screening instruments work, directly or indirectly, on diamond type. GIA says colourless to near-colourless lab-grown diamonds are type II, meaning they have no detectable nitrogen, while only about 1% of natural diamonds are type II; the overwhelming majority of natural diamonds in the D to N colour range are type Ia, which show aggregated nitrogen. Standard screening tests include infrared spectroscopy, ultraviolet transparency and detection of a defect called N3. GIA notes that most screening tools on the consumer market give a "pass" or "refer" result for colourless to near-colourless diamonds: "pass" indicates natural, and "refer" means the stone may be natural or lab-grown and needs further testing. Some devices may not refer simulants, wrongly passing them as natural, so a jeweller has to know the limits of the machine. GIA’s own testing devices include the iD100, an instrument for automated testing of very small diamonds, and DiamondCheck. A referred stone then goes through deep-ultraviolet fluorescence imaging (GIA uses the DiamondView instrument), which commonly shows the growth layers of CVD diamonds and the growth sectors of most HPHT diamonds, and through photoluminescence and infrared spectroscopy. No single marker settles it. A silicon-related line near 737 nm has long been a consistent sign of CVD growth, but in the CVD diamonds GIA examined, its average concentration fell a hundredfold over ten years, and GIA now sees more CVD stones in which it is very weak or undetectable. The same GIA article also describes hybrid stones made in a 2023 study, natural type Ia diamonds with a colourless CVD layer grown on top: they passed standard screening tests, and it took deep-UV imaging and photoluminescence to identify them. Small stones take longer: GIA says a tiny melee diamond can take two to three times longer to analyse than a half-carat one.
How do the inscription and report tell you?
For most buyers, they are the practical answer. Under the lab-grown service GIA announced for 1 October 2025, for D-to-Z lab-grown diamonds, each submitted stone’s girdle is laser-inscribed with "Laboratory-Grown" and its GIA quality assessment number, and a printed document with the results is returned with each stone. IGI says it screens every stone to determine natural, laboratory-grown or simulant origin; its lab-grown diamond report confirms the laboratory-grown origin, and may note the growth method (HPHT or CVD) and any treatments. IGI’s glossary says an inscription is usually the laboratory’s initials and the report number, and IGI says an inscribed report number allows fast identification under magnification, so a jeweller can read it with a loupe or microscope. GIA also says a "LABORATORY-GROWN" inscription or a maker’s mark is commonly found on the girdle of commercial-size lab-grown diamonds, and it has seen CVD stones whose logos and numbers showed only under deep-UV light. An inscription is only as good as the report behind it: GIA examined a moissanite that carried a fake GIA inscription matching the report number of a real natural diamond, and caught it because standard testing showed it was not diamond and its measurements did not match. So look the number up on the laboratory’s own website and compare the measurements: see how to check a diamond report number, and IGI or GIA for what each report gives you.
Frequently Asked Questions
Will a jeweller know if my diamond is lab-grown?
Not by looking, in most cases. A jeweller with a screening device can pass most mined colourless diamonds as natural, but a stone the device refers has to go to a laboratory. If the diamond has a GIA or IGI report, the girdle inscription and the report number answer the question quickly.
Does a diamond tester detect lab-grown diamonds?
No. A thermal diamond tester measures how well the stone conducts heat, and a lab-grown diamond is diamond, so it reads as diamond. Separating lab-grown from mined needs a screening device built for the job, or a gem laboratory.
Can a magnet tell if a diamond is lab-grown?
Only sometimes. GIA says HPHT lab diamonds with larger metallic inclusions can sometimes be picked up with a magnet. GIA also says CVD diamonds do not have metallic inclusions, so a diamond that ignores a magnet can still be lab-grown.
Are lab-grown diamonds always inscribed?
No. GIA inscribes "Laboratory-Grown" and its number on each lab-grown diamond it assesses, and IGI inscriptions usually carry the report number, but a stone that was never sent to a laboratory may carry no inscription at all. An inscription can also be faked, so check the number on the laboratory’s own website.
Sources
- GIA, Gems & Gemology (Summer 2024): Laboratory-grown diamonds, an update on identification and products evaluated at GIA (type II screening, pass/refer, SiV, deep-UV imaging, hybrids, inscriptions)
- GIA: HPHT and CVD diamond growth processes (identification features, magnets, melee, iD100 and DiamondCheck)
- GIA: GIA launches updated laboratory-grown diamond services October 1 (26 August 2025)
- GIA, Gems & Gemology Lab Notes (Fall 2020): Synthetic moissanite with fraudulent GIA inscription (thermal conductivity, measurements)
- IGI: Lab grown diamond reports (screening, growth method, Laserscribe)
- IGI: Glossary (laser inscription)
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