Diamond Fluorescence

Some diamonds glow when exposed to ultraviolet light, the invisible part of sunlight and the light from a UV lamp. That glow is fluorescence. It is usually blue, occasionally yellow, white or another colour, and it is caused by traces of other elements, most often nitrogen, in the crystal.

A good proportion of natural diamonds show some fluorescence. Most of the time you would never know: indoors and under ordinary lighting there is very little UV, so the effect is invisible. You would see it under a UV lamp, in a nightclub, or in strong direct sunlight.

Grading laboratories describe fluorescence by its strength and, when present, its colour:

Fluorescence is not a measure of quality. It is a description, like the colour of a person’s eyes.

Blue fluorescence sits opposite yellow on the colour wheel. In a near-colourless diamond with a faint yellow tint, typically in the I to K range, medium to strong blue fluorescence can make the stone look whiter in daylight. Many people see that as a bonus.

Because the market has historically treated fluorescence with suspicion, natural diamonds with it, especially in the higher colour grades, are often priced a little lower than otherwise identical stones without it.

In a small number of diamonds with very strong fluorescence, the stone can look slightly hazy or milky, sometimes called overblue. This is uncommon, and it cannot be predicted from the report alone. It is the reason to look at the stone itself, in daylight, before you commit.

In the top colour grades, D to F, the slight blue cast that strong fluorescence can give in sunlight may work against an icy white look. Faint or none is the usual preference there.

Do not rule out a diamond because of fluorescence. Judge it by eye. In near-colourless stones, faint to medium fluorescence is rarely noticeable and can be a quiet advantage. With strong or very strong, look at the stone in daylight and from the side to make sure it is bright and transparent. If it is, the grade on the report is just a line on paper.