What fluorescence is

Roughly a quarter to a third of natural diamonds glow under long-wave UV light, as contained in sunlight and some lamps. The cause is traces of nitrogen and other elements in the crystal lattice that absorb UV radiation and emit it as visible light. In the great majority of fluorescent stones this light is blue; more rarely yellow, white or orange.

GIA states fluorescence in five grades: None, Faint, Medium, Strong, Very Strong. The "Fluorescence" filter in our configurator narrows by these grades. For Medium and stronger the report also names the colour of the glow, for example "Medium Blue".

When you see it

Under ordinary indoor light without a UV component, fluorescence is not visible. In direct sunlight, under black light in clubs or under some fluorescent tubes, a strongly fluorescent stone can light up bluish. That is an effect of the light, not a change in the stone.

When it matters – and when it does not

Blue fluorescence can work in two directions:

  • In stones with a slight yellow tint (about I–J and lower) blue fluorescence can offset the tint in daylight – the stone looks whiter than its colour grade suggests.
  • In colourless stones (D–F) with strong or very strong fluorescence, a milky or oily haze can occur in rare cases. In a 1997 study GIA found that most observers, trained ones included, could not tell fluorescent from non-fluorescent stones under normal viewing; the haze affects a small share of strongly fluorescent stones.

Faint and Medium are unobtrusive in practice. With Strong and Very Strong, a look at the stone in daylight or at a daylight photograph is worthwhile.

What the trade makes of it

In the trade, stones without fluorescence are often preferred and strongly fluorescent colourless stones viewed more critically. That is a market convention, not a quality statement by the laboratory. For the buyer what counts is how the specific stone looks – and that can be checked.

Fluorescence as an identification feature

Fluorescence helps in identifying stones: a diamond with "Strong Blue" on its report must glow blue under UV light. Gemmologists use this as one of several features to match stone and report. Laboratory-grown diamonds frequently fluoresce differently (orange, for example, or in patterns), which helps tell them apart – a report from an independent laboratory states in any case whether the stone is natural.

How laboratories determine fluorescence

The laboratory places the stone under a long-wave UV lamp (365 nanometres) and compares the strength of the glow with reference stones. That yields the grade from None to Very Strong; the colour of the glow is noted from Medium upwards. Short-wave UV, as used in some testing devices, produces different reactions and is not used for the report entry. The entry is reproducible and is one of the characteristics by which a stone can be identified.

Fluorescence in photographs and in a setting

In product photographs fluorescence cannot be seen, because studio light usually contains no UV. A photograph in daylight or outdoors shows better whether a strongly fluorescent stone looks bluish. In a setting nothing changes about the property; a strongly fluorescent stone still glows in the sun, which some wearers find attractive.

In short

  • Is fluorescence a defect? No, a property. It is on the report so buyers know about it.
  • Does fluorescence change over time? No. It is built into the crystal and stays.
  • Do all diamonds glow? No; roughly a quarter to a third of natural stones, most of them faintly.
  • Can fluorescence be removed? No, not without altering the stone.
  • Should I avoid stones with fluorescence? For Faint and Medium there is no practical reason. For Strong and Very Strong: look at the stone.

Fluorescence in coloured diamonds

In fancy colours fluorescence plays a different role than in colourless stones. Blue fluorescence can make a yellow fancy stone look slightly cooler in sunlight; in pink stones the glow is often orange or bluish and hardly noticeable. The laboratory states fluorescence on the Colored Diamond Report just as on the report of a colourless stone. It is not used for the colour grading itself; colour is judged under standard light without a UV component.

Comparing fluorescence in photographs and videos

Anyone comparing two stones with different fluorescence should view both under the same light: once under a lamp without UV (most LED lights), once at a window in daylight. Only the second look shows whether the entry on the report affects the appearance. A short video in daylight says more than any studio photograph – on request we provide one for a specific stone.

What our stone pages show

Every stone's page shows fluorescence exactly as the report states it, and the configurator filter can be set by grade. Anyone unsure about a colourless stone with strong fluorescence can view it at our office in Dubai or have it checked after receipt within the statutory withdrawal period.