Diamond Fluorescence Explained: Should You Worry?

Fluorescence appears on a diamond certificate as one short line, and it sometimes causes unnecessary worry. Here is what it means.

What is fluorescence?

Some diamonds give off a soft glow, most commonly blue, when exposed to ultraviolet (UV) light. It is the same effect that makes certain white clothes glow under party lights. It is caused by trace elements within the diamond's structure.

How is it graded?

Laboratories describe the strength of the reaction:

• None
• Faint
• Medium
• Strong
• Very Strong

The colour of the glow is noted, usually blue.

Does it change how a diamond looks?

In most stones, no. Under ordinary indoor and outdoor lighting, a diamond with faint or medium fluorescence looks the same as one with none. Strong blue fluorescence can in some cases make a stone look slightly hazy or oily, but this is uncommon and is noticed only in certain stones. In other stones it can make a slightly yellow diamond appear whiter in daylight, which can be a positive.

Does it affect price?

Fluorescence can affect price. Stones with strong fluorescence are sometimes priced lower than comparable stones with none, particularly in higher colour grades, because some buyers prefer to avoid it. For others this is an opportunity: if a stone with faint to medium fluorescence looks excellent to you, you may pay less for it.

Is it a defect?

No. It is a characteristic, like the other items in the report, and it is neither good nor bad in itself. What matters is how the particular diamond looks to you.

How to decide

1. Look at the stone in shop light, daylight and, if possible, under a UV lamp.
2. Check whether it looks hazy or dull face-up. If it looks clear and bright, the fluorescence is not hurting it.
3. Compare it with a stone with no fluorescence.
4. Consider the price benefit, if there is one.

Fluorescence in small diamonds

For the small stones in a pendant or pair of studs, fluorescence rarely matters. It is more of a consideration for solitaires.

Where it appears on the report

Fluorescence is its own entry on the report. We explain the other entries in how to read a diamond certificate step by step.

Is a glowing stone fake?

No. Plenty of natural diamonds fluoresce. Fluorescence cannot be used on its own to tell a natural diamond from an imitation, as discussed in how to spot a fake diamond.

Common questions

Does fluorescence fade? No. It is a stable property of the stone.

Should I avoid strong fluorescence? Not automatically. Look at the stone. If it looks clear and bright to you, the grade is a point of price, not of beauty.

Can fluorescence be seen in sunlight? Daylight contains some UV, so a stone with strong fluorescence may show a subtle effect outdoors.

A note on UV lamps in shops

Some showrooms keep a small ultraviolet lamp so that buyers can see a stone's reaction. This is helpful for curiosity, but it is not how you will wear the diamond. Make your decision on how the stone looks in ordinary light, and treat the UV glow as extra information rather than a verdict.

Judging for yourself

The best test is your own eye. You can book a visit to compare stones, or explore our diamond rings and ask about the details of any piece that interests you.