What makes some diamonds glow?

An intriguing phenomenon of some diamonds is that they appear to glow blue, and less often green, yellow, orange, or a combination of these colors when exposed to invisible ultraviolet rays. This glow is called fluorescence, and lasts only as long as the diamond is exposed to the ultraviolet rays.

The culprit nitrogen

In most cases, fluorescence is caused by the presence of nitrogen as a trace element impurity in diamond. You may know that diamonds are composed of carbon atoms organized in a lattice-like crystal structure. Sometimes some carbon atoms are missing in the lattice. These single or multiple vacancies may each be filled by a nitrogen atom. For example, a single nitrogen atom trapped near a vacancy causes bright orange-yellow fluorescence. But most commonly, three nitrogen atoms in lattice positions adjacent to a vacancy cause blue fluorescence.

In addition to color, fluorescence also varies by strength – from none, faint, medium, strong, and very strong, as described on GIA grading reports. The fluorescence color and its intensity are additional characteristics that can help to identify a specific diamond.

Fluorescence in Coloured Diamonds

The glow caused by the effect of fluorescence, coupled with the diamond's colour will often work to complement each other. Under the right lighting, diamond fluorescence can actually help enhance the natural beauty of a fancy coloured diamond. Additionally, the fluorescence won’t reduce the colour intensity of the stone in fancy coloured diamonds to any significant degree.

Finding a yellow diamond with fluorescence is actually quite common. The level of illumination however is usually quite faint. Pink diamonds for example, are quite difficult to find without fluorescence.

With pink diamonds, unlike other colourless and some coloured diamonds, since the addition is so common, especially in Argyle pink diamonds, even a strong grade will generally not negatively impact the value of the stone. On the other hand, a pure blue diamond will almost always not have any fluorescence at all. In the event that fluorescence is found, it most likely means that the diamond is slightly combined with a greenish overtone but the fluorescence will usually not even be noticeable.

Certain diamond fluorescence are extremely rare and actually considered collector's items, for example, a red fluorescence. Whether positive or negative, to see the affect in reality is breathtaking.

A common feature of diamonds

Fluorescence is a common phenomenon in diamond. Of all the diamonds submitted to GIA over the past decade, approximately 25% to 35% exhibit some degree of fluorescence. And of those, only 10% show strengths of fluorescence in the medium, strong, or very strong categories.

GIA studies have shown that the strength of fluorescence has no widely noticeable effect on a colorless or near-colorless diamond’s appearance. In fact, many prefer the appearance of diamonds that have medium to strong fluorescence. In very rare cases (fewer that 0.2% of the fluorescent diamonds submitted to GIA), some diamonds with extremely strong blue fluorescence may appear hazy or oily.

No impact on a diamond’s strength

The presence of nitrogen or other impurity atoms in the diamond crystal structure does not have any influence on the hardness or durability of a diamond.

Source - The Gemological Institute of America