Do lab-grown diamonds glow under UV light?

Quick answerUpdated October 17, 2026

Some lab-grown diamonds glow under UV light and some do not. The more telling sign is a glow that lingers after the lamp goes off, called phosphorescence: GIA says colourless HPHT lab diamonds generally show an intense, long-lived blue-green afterglow, while only a few natural diamonds react the same way. It is a clue, not a test; a laboratory report settles it.

What is the difference between fluorescence and phosphorescence?

is a glow you see while the ultraviolet light is on. Some diamonds fluoresce in long-wave UV, the kind that comes from sources such as the sun. Phosphorescence is light the stone keeps giving off after the UV is switched off. In diamond the two tell you different things. Plenty of mined diamonds fluoresce: GIA says about 25% to 35% of diamonds glow to some degree under long-wave UV, and more than 95% of those glow blue. A long afterglow is another matter. GIA says it is often used as a key indicator for colourless and near-colourless HPHT lab diamonds (grown at high pressure and high temperature), which generally show an intense, long-lived blue-green phosphorescence. So do lab-grown diamonds glow in the dark? Not on their own. They need UV first, and GIA says some then glow for a minute or more after the lamp is turned off.

Do lab-grown diamonds have fluorescence?

Some do, but often in a different way from mined stones. GIA says lab diamond fluorescence is often stronger under short-wave than under long-wave UV, and that the opposite is true of mined diamonds that fluoresce. A particular lab diamond may show no fluorescence at all. Under a laboratory deep-UV imaging instrument the growth method shows. HPHT diamond grows in distinct sectors, and GIA’s images show a cross-shaped fluorescence pattern on the crown or pavilion. CVD diamond (chemical vapour deposition), built up layer by layer, shows striped growth layering. Colour matters too: a treated orangy pink CVD stone in GIA’s 2024 report glowed red under deep UV, from a high concentration of nitrogen-vacancy centres. GIA links the HPHT afterglow to trace amounts of boron. Both growth methods are explained in how lab gems are grown.

How do lab-grown and mined diamonds compare under UV?

Diamond	365 nm torch (long-wave UV)	Laboratory short-wave and deep UV	Afterglow when the lamp goes off
Mined	About 25% to 35% glow, more than 95% of those blue; the rest show nothing	Fluorescent mined diamonds often glow more under long-wave than short-wave	Rare: only a few natural type IIb diamonds react like HPHT lab diamonds
HPHT lab-grown	Often weaker than under short-wave; may show nothing	Cross-shaped pattern from its growth sectors	Colourless and near-colourless stones generally show an intense, long-lived blue-green glow; low-dose irradiation can reduce or remove it
CVD lab-grown	Often weaker than under short-wave; may show nothing	Striped growth layering; some stones look nominally like mined ones, so labs add other tests	Some CVD stones show a comparable glow
What it proves	Nothing on its own	Read by a lab together with spectroscopy	A clue that points toward HPHT, not proof

What does a 365 nm UV torch show?

A 365 nm torch gives long-wave UV: the WHO puts UV-A, the longest-wave band, at 315 to 400 nm. In a 2021 study, GIA researchers measured the blue glow of research diamonds under a 365 nm UV source, and the readings matched the Faint and Strong fluorescence descriptions on the stones’ GIA reports. So a filtered 365 nm torch, like the one in our gem tools guide, shows the same kind of long-wave glow that the fluorescence line on a diamond report describes. To look for an afterglow, work in a dark room, shine the torch on the stone, switch it off and keep watching. A blue glow while the torch is on is the usual reaction of mined diamonds that fluoresce. No glow tells you little, because most mined diamonds show none and a lab-grown one may show none too. A blue-green glow that carries on after the torch is off points toward HPHT growth. A torch may still miss it: GIA’s 2024 examples showed their afterglow after deep UV, and lab diamond fluorescence is often weaker under long-wave UV. Never shine UV into anyone’s eyes. Short-wave lamps, around 254 nm, give off UV-C, which the WHO calls the most damaging type of UV radiation; keep them away from eyes and skin.

Can UV light tell a lab-grown diamond from a mined one?

Not on its own. GIA warns that relying on a phosphorescence observation, or on any other single observation, is not advisable when testing colourless HPHT lab diamonds. Low-dose irradiation can reduce or remove the afterglow, a few natural type IIb diamonds and some CVD stones glow in a similar way, and a lab diamond may not fluoresce at all. By eye, GIA says, the two can be indistinguishable. Many screening methods start from diamond type: all colourless lab-grown diamonds are type II, while only about 1% of natural diamonds are. Laboratories then use photoluminescence spectroscopy, deep-UV imaging and, occasionally, cathodoluminescence imaging when a deep-UV pattern is unclear. What to do instead: buy a lab diamond with an IGI or GIA report number and look it up (see how to check a diamond report number and IGI vs GIA for lab-grown diamonds). GIA laser-inscribes the girdle of each lab-grown diamond it reports on with the report number and a statement that the stone is lab-grown.

Frequently Asked Questions

Do lab-grown diamonds glow in the dark?

Not by themselves. After exposure to ultraviolet light, colourless HPHT lab diamonds generally keep glowing blue-green for a while, and GIA says some lab diamonds glow for a minute or more after the lamp is turned off. Without UV first, a diamond does not glow.

Is a diamond that does not glow under UV lab-grown?

No. GIA says only about 25% to 35% of diamonds fluoresce under long-wave UV, so most mined diamonds show no glow, and a lab-grown diamond may show none either.

Does blue fluorescence mean a diamond is natural?

No, although it is common in mined diamonds: GIA says more than 95% of diamonds that fluoresce glow blue. Lab diamonds can fluoresce too, often more strongly under short-wave than long-wave UV, and GIA reports some CVD lab diamonds whose deep-UV fluorescence closely resembles that of natural diamonds. The glow alone does not settle where a diamond came from.

Can UV light change a lab diamond’s colour?

Temporarily, in some CVD stones. GIA reports that UV exposure can darken the body colour of some CVD lab diamonds, and that these changes are temporary and fully reversible: white light restores the original colour.

Does fluorescence make a diamond look hazy?

Not by itself, GIA says. A 2021 GIA study found that a hazy look comes mainly from light scattering by structural defects, with strong fluorescence adding only minor contrast loss in some stones.

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