Fluorescence
Fluorescence is the visible glow, usually blue, that some diamonds emit under ultraviolet light, graded from None to Very Strong.
Fluorescence is the soft glow a diamond gives off under ultraviolet light, most often a blue tone. GIA records it on a report as an identifying feature, from None through Faint, Medium, Strong and Very Strong. It is not one of the 4Cs, but it can nudge both how a stone looks and what it sells for.
What causes it and how it is graded
Roughly a quarter to a third of diamonds fluoresce to some degree, caused by trace elements in the crystal reacting to UV. Sunlight and some indoor lighting contain UV, so a strongly fluorescent stone can glow faintly in daily life, though usually you would need a UV lamp to see it clearly. GIA notes the intensity and, if present, the colour, which is blue in the great majority of cases.
The grade describes strength, not quality. None means no reaction, while Very Strong means a vivid glow under UV. Because most fluorescence is not visible in normal viewing, GIA treats it as an identifying characteristic that helps distinguish one diamond from another rather than a measure of beauty.
When fluorescence helps and when it hurts
In faint yellow diamonds, grades in the I to M range, blue fluorescence can make the stone look whiter, because blue counteracts the yellow tint. That can be a genuine bonus, and such stones sometimes trade at a small discount for a benefit you can actually use.
The concern is at the top of the colour scale. In a small share of Strong and Very Strong stones, usually D to F, the diamond can look slightly hazy, milky or oily in strong UV or sunlight. GIA research found that for most stones fluorescence has no visible effect, but this cloudy minority means high-colour stones with strong fluorescence often sell at a discount.
- Blue fluorescence can whiten faint yellow diamonds (I to M)
- Strong fluorescence in D to F stones occasionally looks hazy
- Most fluorescent diamonds look no different in normal light
- Strong grades usually trade at a discount
- Fluorescence is a glow, usually blue, seen under UV light.
- GIA grades it None, Faint, Medium, Strong or Very Strong.
- It is an identifying feature, not one of the 4Cs.
- Blue fluorescence can make faint yellow diamonds look whiter.
- Strong grades in high-colour stones can occasionally look hazy and trade lower.
Fluorescence, in short.
Is diamond fluorescence bad?
Usually not. GIA found most fluorescent diamonds look no different in normal light, and blue fluorescence can even improve faint yellow stones. Only a small share of strong-fluorescence high-colour diamonds look hazy.
Does fluorescence lower a diamond's value?
It can. Strong and Very Strong fluorescence often sells at a discount, which can be a saving if the stone still looks good to your eye. In faint yellow stones the discount is essentially free value.
Can I see fluorescence in everyday light?
Rarely. You normally need a UV lamp to see it clearly. Sunlight carries some UV, so very strong stones might glow faintly outdoors, but most fluorescence is invisible in daily wear.
Related terms
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