We are increasingly asked about fluorescence by customers who are looking for a diamond. Routinely mentioned on certificates though rarely spoken about by salespeople, fluorescence can make a significant difference to the price and sometimes the appearance of a diamond, but it is a tricky subject and it can be difficult to pin down exactly what its impact on a stone can be.

Fluorescence is an effect seen when a material emits visible light under exposure to high energy radiation such as ultraviolet (UV) light.   Around 25-35% of diamonds fluoresce to some degree. The fluorescent colour is usually blue, but it can also occasionally occur in other colours. The intensity of the glow can also vary, with some diamonds exhibiting a very strong fluorescence while others have only a weak glow. On a Gemological Institute of America (GIA) certificate the level of fluorescence will be stated as None, Faint, Medium, Strong or Very Strong. For Medium to Very Strong fluorescence the colour will also be noted, as this can alter the effect of the fluorescence on the appearance of the stone. For an example of the massive variation possible just take a look at the two images below: they are of the same pendant, one in natural light and one under UV light.

Fluorescence can have a positive, negative or no impact on the appearance of a diamond to the eye. For stones with a faint yellowish body colour, perhaps graded I to M (see our blog on colour for more), the presence of blue fluorescence can cause the colour to appear whiter than a similar stone with no fluorescence. This is because blue and yellow are complementary colours; when one is removed from white light you are left with the other. Natural daylight and many electric lights contains an amount of UV radiation, causing enough of a fluorescent effect to alter the balance of blue to yellow without giving the stronger blue glow that is seen under a special UV light source. With diamonds of a higher colour grade the advantages of this “whitening” effect diminishes and fluorescence becomes increasingly undesirable.

The problem with fluorescence is that in some cases it can cause a diamond to appear hazy or milky, especially if the fluorescence is strong.   If the fluorescence has a positive impact on the colour of a stone an amount of haziness is sometimes forgivable, but with finer stones or those where the colour is not improved it is generally better to avoid a property which can lessen the stone's attractiveness.  Stones which fluoresce are usually less expensive than those which do not, all other factors being equal, so it is worth checking the detail before making a purchase.

In the above diamonds the variation in appearance is significant. Both are the same size, colour grade and clarity grade and both have very strong blue fluorescence.  The stone on the left has a very hazy appearance while the stone on the right does not.  Without seeing the diamond or being told of this difference by the seller it wouldn’t be possible to infer it from the certificate, but the hazy stone is considerably less expensive so would sound the best value on paper yet in reality could disappoint.

Despite extensive research experts have found it to be virtually impossible to accurately predict and quantify the impact of fluorescence on the appearance of a diamond. Only viewing a stone in person and in various lighting conditions can really provide an answer. Even seeing photographs is no substitute for seeing the diamond as it isn’t possible to say if a hazy appearance is due to the diamond or the photography or if the diamond has been photographed in complementary lighting.

Regardless of the challenge it poses, fluorescence is fascinating, and can be incredible to view like in the picture of the pendant above. Because the property varies so much in diamonds it is possible to see a wide range of effects. Jewellery with clusters of diamonds will usually display a variety in fluorescence across the piece with some stones remaining inert under UV while others glow with varying levels of intensity. Sometimes diamonds surprise us, fluorescing with an unusual colour or simply having an unusually bright glow. It can be fascinating to observe the variation between different stones.

Once you understand fluorescence and its affects it's another quality to consider when finding the right diamond. If you are sensitive to a tinted colour or eye visible inclusions but wish to find a particular size to fit into your budget then accepting a diamond with noticeable fluorescence may be the best option for you, as always it’s about making an educated and considered choice. Perhaps more than any other quality if you choose a diamond with fluorescence it’s important to have trust in your jeweller to advise you on its impact on the beauty of the diamond and to compare with another diamond that doesn’t fluoresce, as you can’t rely on a diamond report to guide you.

If you have any questions about diamonds including fluorescence or even if you would just like to see the fluorescence for yourself, our qualified gemmologists and diamond graders are always available in the showroom.