Diamond fluorescence: does it matter in an engagement ring?

Diamond fluorescence is the soft glow, most often blue, that some diamonds give off under ultraviolet light, and a grading report describes its strength on a scale from None through Faint, Medium and Strong to Very Strong. Whether it matters in an engagement ring depends on that strength and on the stone's colour grade, and in most rings it makes no difference you would notice.

It is a term buyers meet on grading reports and in diamond listings, often without explanation, and it can raise a worry it does not deserve.

What is diamond fluorescence?

Fluorescence is visible light that a material gives off while it is exposed to ultraviolet. In a diamond it comes from minute irregularities in the crystal structure, including atoms of other elements held within it. Depending on those irregularities, some diamonds react and most do not.

The glow is usually blue. More rarely it is yellow, green or another colour. It lasts only as long as the ultraviolet source is present. Daylight carries long-wave ultraviolet, and so do the blacklights used in some bars and clubs, under which a strongly fluorescent diamond can glow a vivid blue.

Fluorescence has no bearing on durability. The Gemological Institute of America (GIA) states that a diamond which fluoresces has the same structural integrity as one that does not react to ultraviolet at all.

How is fluorescence graded on a report?

Fluorescence is not one of the 4Cs. GIA records it on the report as an identifying characteristic rather than a grading factor, and states that it does not affect the colour grade. If the 4Cs themselves are new to you, our guide to the 4Cs of diamonds sets out how each one is judged.

GIA assesses the strength of a diamond's reaction under a controlled long-wave ultraviolet lamp and describes it in five steps:

• None: no reaction seen under the laboratory's lamp.
• Faint: a slight reaction. The colour of the glow is not named.
• Medium: a clear reaction, with its colour named, for example Medium Blue.
• Strong: a pronounced reaction, again with its colour named.
• Very Strong: the most intense reaction the scale describes.

IGI reports record fluorescence as well, in similar terms. The two laboratories differ in other ways, which we cover in how GIA and IGI diamond reports compare. On either report, this line records how the stone behaves under a laboratory lamp; how it looks on a hand in daylight or across a dinner table is a separate question.

Can you see fluorescence in normal light?

In everyday light you usually cannot. Most indoor lighting carries much less ultraviolet than daylight, so indoors a fluorescent diamond looks like any other. Outdoors in daylight, the ultraviolet can draw a response from a stone with Medium or stronger fluorescence, though the effect is generally subtle.

GIA has tested this directly. Its researchers assembled sets of diamonds matched as closely as possible in every respect except the strength of their blue fluorescence, and asked diamond graders, trained professionals and ordinary observers to judge them under controlled conditions. For the average observer, standing in for the jewellery-buying public, no systematic effect was found. Viewed face up, the strongly blue diamonds were in general judged to have the better colour, and most observers saw no link between fluorescence and transparency. GIA sets out the study and its advice in its own guide to whether fluorescence is good or bad.

Is fluorescence bad in a diamond?

GIA's answer is that fluorescence is neither good nor bad, and that liking it or not is a personal preference. It does not make a diamond fragile, and GIA's studies found that for the overwhelming majority of diamonds it has no widely noticeable effect on appearance.

Its poor reputation comes from haziness. Some in the trade believe that strong blue fluorescence can make a diamond look milky or oily. GIA's research found that fluorescence does not cause haziness on its own. In extremely rare cases, a diamond with a separate light-scattering defect can look hazy, and strong fluorescence can deepen that effect, which would show in daylight or daylight-equivalent light. That is a reason to look closely at the stone before ruling out a grade.

The reputation still shows in pricing. GIA notes that diamonds in the D to H colour range with very strong fluorescence often sell for less than comparable stones without it, even though their colour and transparency are most likely unaffected. For a buyer, that can mean a lower price for a stone that most likely looks no different.

Does faint fluorescence matter?

Faint fluorescence very rarely makes a visible difference. It is the gentlest reaction a report records. GIA counts only Medium, Strong and Very Strong among the strengths that may affect appearance, and its observers found no systematic effect even across those, so the step from None to Faint is not one a person wearing the ring would expect to see.

When comparing two otherwise similar stones, it is sensible to treat None and Faint as equivalent and let cut, colour and clarity decide between them. Some people still prefer None for peace of mind, particularly in the top colour grades. We would not give up a better cut for it, though.

Fluorescence and colour grade

Read beside the colour grade, the same word can be good news on one report and a question on another. Colour in a white diamond is graded from D, the most colourless, down towards Z, as explained in our guide to how diamond colour is graded. The faint warmth that appears lower down the scale is usually yellowish, and blue is its complementary colour.

In stones graded around I to M, which can carry a trace of warmth, blue fluorescence can make the diamond look a little more colourless in natural daylight. Some trade professionals see this as a benefit at those grades. The colour grade on the report does not change, because the laboratory grades colour in a controlled environment designed for consistency.

In the colourless and upper near-colourless grades, D to H, there is little warmth for blue to offset. Here the trade has tended to prefer stones without fluorescence, which is the source of the price difference described above. A D to H stone with Strong Blue fluorescence that looks bright and clear in daylight is no lesser a diamond to the eye.

So for Medium or Strong Blue, look at how it sits with the colour grade, the metal and the light the ring will live in. Yellow and rose gold are more forgiving of a touch of body colour than platinum or white gold.

Do laboratory-grown diamonds fluoresce?

Laboratory-grown diamonds can fluoresce too. Fluorescence is a property of the crystal, and a laboratory-grown diamond has essentially the same chemical, physical and optical properties as one formed in the earth. Where a report on a laboratory-grown diamond records fluorescence, read it as you would for a mined stone.

Grading laboratories also study how a stone reacts to ultraviolet, among many other tests, when they confirm whether a diamond was grown or mined. That is specialist laboratory work that cannot be judged by eye, and it is one of the questions an independent report settles. A GIA or IGI grading report is available on the centre stones we source. Our pages on laboratory-grown diamonds and natural diamonds cover each in more depth.

How we weigh fluorescence when sourcing your stone

Our Hatton Garden studio sits inside The Goldsmiths' Centre and works by appointment. There is no loose stock to browse, so we source natural and laboratory-grown diamonds for each commission and you choose from the reports, images and video of the stones we find.

When a stone makes your shortlist, we take you through its report, fluorescence included, and explain what that line means for that stone at its colour grade. Images and video of the actual diamond are part of that choice, and where the report shows Medium or stronger fluorescence they are worth studying closely, so you can see how bright and clear it looks. When a stone carries a GIA or IGI report, the description you are reading is an independent one.

If the stone is what matters most to you, you can start with the diamond and build the ring around it. A complete bespoke ring starts from £2,000, and most of our commissions come to between £3,000 and £12,000. We hold consultations in the studio, by phone or by WhatsApp, and we typically reply within four working hours, Monday to Friday.

To go through a stone and its report with us, book a consultation.

Common questions

Does fluorescence damage a diamond over time?

GIA states that a fluorescent diamond has the same structural integrity as one with no reaction to ultraviolet. The glow happens only while ultraviolet is present and leaves no mark on the stone.

Will my diamond glow under a blacklight?

If the report shows Medium, Strong or Very Strong fluorescence, you may well see a glow, most often blue, under the ultraviolet lights used in some bars and clubs. A stone graded None will show little or nothing.

Does fluorescence change a diamond's colour grade?

No. Colour is graded in a controlled environment, and GIA notes that the colour grade on its report is not affected by fluorescence. The stone's appearance in daylight can be affected, which is why the two lines on the report are best read together.

Should I avoid very strong fluorescence?

You need not rule it out. The haziness it is blamed for is, on GIA's findings, extremely rare and caused by a separate defect rather than by fluorescence itself. Study the images and video of the stone and ask about its transparency before you commit.