An intriguing phenomenon of some diamonds is that they appear to glow blue, and less often green, yellow, orange, or a combination of these colours 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.
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 colour, fluorescence also varies by strength - from none, faint, medium, strong, and very strong, as described on GIA grading reports. The fluorescence colour and its intensity are additional characteristics that can help to identify a specific diamond.
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 colourless or near-colourless 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.
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.
Listed below are the abbreviations for describing strength of fluorescence that should be noted when reading a diamond grading report:
- N, NO, NON, - No Fluorescence
- F, FB, FT, FA, - Faint or Faint Blue Fluorescence
- SL, SLB, SLT, - Slight Fluorescence
- M, MO, ME, MD - Medium or Moderate Fluorescence
- S, ST, STB, STG, - Strong Fluorescence
- E, EX, EXB, EF, - Extreme Fluorescence