Many people have seen a diamond sparkle in sunlight, but fewer realise that some diamonds can actually glow when exposed to ultraviolet (UV) light. If you have ever been in a nightclub with a black light or watched a jeweller examine a diamond under specialist equipment, you may have noticed a blue or sometimes yellow glow coming from the stone.
This effect is called fluorescence. Although some people assume a glowing diamond must be artificial, that is not the case. Fluorescence occurs naturally in many diamonds and has been recognised by gemmologists for decades.
What causes a diamond to glow?
Diamonds form deep within the Earth under intense heat and pressure. As they grow, tiny structural irregularities and trace elements can become locked into the crystal.
When exposed to UV light, some of these features react and emit visible light, creating the glow known as fluorescence.
The effect only lasts while the UV light is present. Once the light source is removed, the glow disappears almost immediately.
Because every diamond forms under slightly different conditions, not all stones fluoresce. Some show a strong reaction, some a very faint one, and many show no fluorescence at all.
How common is fluorescence?
Fluorescence is more common than many people realise with estimates citing around one quarter to one third of natural diamonds showing some degree of fluorescence when examined under ultraviolet light.
The strength of the glow can vary considerably. Some diamonds show only a very faint reaction, while others produce a much stronger glow that is easy to spot under a black light.
Why do most fluorescent diamonds glow blue?
Blue is by far the most common fluorescence colour seen in diamonds.
Gemmologists believe this is linked to the types of structural features most commonly found in natural diamond crystals. Although other colours are possible, including yellow, green, white and orange, blue remains the colour most frequently encountered. The blue glow can help offset faint yellow tones within a diamond, making it appear slightly whiter to the eye.
Can you see fluorescence in everyday life?
Usually not.
Most indoor lighting contains very little ultraviolet light, meaning fluorescence is often invisible during normal wear. Even owners of fluorescent diamonds may never notice the effect.
Bright sunlight contains UV rays, so some fluorescent diamonds may show a subtle reaction outdoors. However, the effect is generally much easier to see under dedicated UV lamps or black lights.
For this reason, fluorescence is often something people discover only after having a diamond examined by a jeweller.
Does fluorescence affect quality?
This is where a lot of myths exist.
Some buyers worry that fluorescence means a diamond is lower quality, but this is not how grading laboratories view it. Often it is considered a characteristic rather than a defect as it does not affect the structural integrity of the diamond.
A small number of diamonds with very strong fluorescence can sometimes appear slightly hazy or milky, but this is relatively uncommon.
Does fluorescence affect value?
The answer depends largely on buyer preference.
Historically, some fluorescent diamonds have sold for slightly less than comparable non-fluorescent stones, particularly when fluorescence is described as strong or very strong.
However, many buyers actively seek fluorescent diamonds because they can offer good value for money. Some also appreciate the unique character that fluorescence gives to a stone.
As with any diamond purchase, factors such as cut, colour, clarity and carat weight typically have a much greater influence on value than fluorescence alone.
Insuring valuable diamond jewellery
A diamond's value is about more than its technical characteristics. Whether your jewellery contains a fluorescent diamond or not, it may be worth considering jewellery insurance. Cover may include worldwide protection against theft, accidental loss and damage, subject to terms, conditions and exclusions.
At Assetsure, we provide specialist jewellery insurance for engagement rings, diamond rings and other valuable jewellery.
A feature with its own story
One of the reasons diamonds continue to fascinate people is that no two stones are exactly alike. Fluorescence is simply another part of that individuality.
For some owners, discovering that their diamond glows under UV light adds another layer of interest to a gemstone that may already hold sentimental value. It is a reminder that diamonds are natural products shaped by geological forces over immense periods of time.
Far from being something to worry about, fluorescence is often just another fascinating chapter in a diamond's story.
Disclaimer: Information is provided for general guidance only. The appearance and characteristics of individual diamonds can vary, and professional advice should always be sought before making purchasing decisions.
