Diamonds aren’t just the colour you see under natural light – they reveal a hidden world of colour under ultraviolet (UV) light! When exposed to UV light, many natural diamonds exhibit fascinating fluorescence, which can range from vibrant yellows to striking blues and even bold oranges, as seen in this image. Did you know that around 25–35% of diamonds glow to some degree?
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What Is Fluorescence in Diamonds?
Fluorescence is the visible glow a diamond gives off when it is exposed to invisible ultraviolet light, such as the long-wave UV found in daylight or the lamps used in nightclubs and at airport security. The glow stops the moment the UV source is removed, which is what separates it from phosphorescence, where a stone continues to glow for a short time afterwards.
The effect is caused by tiny structural features and trace elements that were locked into the crystal as it grew deep within the Earth. These absorb the high-energy UV light and release it again as lower-energy visible light, producing the colours we see.
The Colours of Diamond Fluorescence
Blue is by far the most common colour, accounting for well over 90% of fluorescent diamonds. It is linked to nitrogen-related defects within the crystal lattice and can make a slightly tinted diamond appear whiter and more brilliant under certain lighting conditions.
Yellow is often seen in diamonds with a particular arrangement of nitrogen atoms, giving them a warm, inviting glow. Interestingly, diamonds that glow orange are rarer, creating a unique warmth that adds an unexpected twist to their sparkling allure. Green, white and even red are also seen occasionally, and the colour and strength of the glow are a useful fingerprint for each individual stone.
How the UV Glow Is Graded
Laboratories assess the reaction under a standard long-wave UV lamp and compare it to a set of master stones. Diamonds can display either none, faint, medium, strong or very strong fluorescence, and we record this on every one of our loose diamond reports at E.D.R, alongside the 4Cs of carat, colour, clarity and cut.
The grade describes the strength of the glow, while the colour is noted separately. Because it is a permanent, natural characteristic of the stone, it is also a helpful check when verifying that a diamond matches its report.
Does Fluorescence Affect a Diamond’s Value?
In most cases, the effect on appearance is negligible and can only be seen under UV light. In fact, for diamonds in the lower colour grades (I to M), a medium or strong blue glow can be a bonus, making the stone face up whiter in daylight. Research published by the Gemological Institute of America (GIA) found that most observers could not tell fluorescent and non-fluorescent diamonds apart in normal lighting.
The exception is a very small number of very strong stones that can appear hazy, milky or oily. These are uncommon, but it is the reason strongly fluorescent diamonds in the top colour grades (D to F) sometimes trade at a slight discount. Conversely, this can represent excellent value for a buyer who has examined the stone and is happy with how it looks.
A Useful Identification Tool
Beyond its visual appeal, the UV response of a diamond gives gemmologists valuable clues about its chemical composition and formation history. Here are a few ways we use it in the laboratory:
- Matching stones: diamonds in a pair of earrings or a multi-stone ring should ideally have a similar UV reaction so that they look consistent in all lighting.
- Spotting simulants: cubic zirconia, moissanite and glass react very differently under UV light compared with natural diamond.
- Screening for laboratory-grown diamonds: many laboratory-grown diamonds show distinctive patterns, colours or phosphorescence that natural stones rarely display.
- Confirming identity: a stone’s UV reaction is permanent, so it helps confirm that a diamond matches an existing report.
For a deeper look at how colour and body tint are assessed, see our guides to coloured diamonds and April’s birthstone, the diamond.
So, the next time you’re admiring a diamond, remember that its true magic may just reveal itself when you shine a UV light on it! If you would like your diamond graded, including its fluorescence, contact our London laboratory today.
