Diamond fluorescence

When a diamond is exposed to long-wave UV light, some stones give off visible light. The usual result is a blue glow, and once the UV source is removed the diamond stops fluorescing. In GIA's figures, about 25% to 35% of the diamonds submitted to it show some fluorescence, and more than 95% of those glow blue. A rare few glow another colour, such as yellow or green. GIA also says that only about 10% of the fluorescent diamonds show enough of it to affect their appearance.

Fluorescence is not part of the 4Cs. GIA treats it as an identifying characteristic of the stone, which is why it appears on the diamond grading report without feeding into the colour grade or any of the other three.

A GIA report uses five terms for the strength of the glow. When the strength is Medium, Strong or Very Strong, GIA also notes the colour of the glow, so a line reading "Strong Blue" tells you both. For None and Faint there is no colour to read.

Look for both pieces of information. Because more than 95% of fluorescent diamonds glow blue, a report that names another colour describes an unusual stone, and it is worth asking about. The strength term is only a label for the glow under UV light. It says nothing about the quality of the diamond, which is why it sits apart from the 4Cs grades.

The haze worry is the one most buyers hear first. Fluorescence does not cause haziness by itself. Haziness comes from certain internal features, and strong fluorescence can sometimes intensify it by reducing contrast in the face-up pattern. That happens in fewer than 0.2% of the fluorescent diamonds submitted to GIA. GIA's quality-factor page adds that if the fluorescence is too strong, the stone can look cloudy or oily. It is rare, but it is the reason to look at any Strong or Very Strong stone before you buy it.

Colour is where fluorescence can help. Bluish fluorescence can make a faint yellowish diamond look more colourless in natural daylight, and some trade professionals think blue fluorescence improves the look of diamonds graded I to M. The colour grade itself is not affected, because GIA engineers its grading environment for consistency.

GIA also ran observer studies. Its finding was that average observers, meant to represent the jewellery-buying public, could not consistently tell fluorescence-related effects apart in conditions close to those in which jewellery is bought and worn. Viewers generally judged strongly blue fluorescent diamonds to have better colour when looking at them table-up.

GIA's position is that fluorescence is neither good nor bad. Opinion in the trade is divided on value. Some professionals believe the very rare stones at the top of the colour scale with extremely strong blue fluorescence are worth less because they can look hazy or milky, and diamonds in the D to H range with very strong fluorescence often sell for less than ones without it. That is a statement about the market, not a defect in the stone, and it could affect what another buyer would pay for the stone later.

A practical way to decide:

• Read the report for the strength and, from Medium upward, the colour.
• View the diamond in daylight and under indoor light, face-up, and ask whether the stone looks clear and lively to you.
• Check Strong and Very Strong stones carefully for any milky or oily look, remembering how rare it is.
• Compare it against a stone with None or Faint, then choose the one you prefer to look at.

If the stone is for a custom engagement ring, a showroom visit or the design studio is the place to talk through it.