Fluorescence is a form of luminescence or light emission under ultraviolet light excitation. In the case of the day-to-day diamond trade, it is mostly experienced when checking diamonds under longwave UV lamps. However, it may also be seen under disco lighting conditions or more rarely when stones are exposed to sunshine. It causes diamonds to exhibit a soft glow which may appear white or light blue and more exceptionally yellow and rarely orange, pink, red or green.
Phosphorescence, on the other hand, may be distinguished as an afterglow. Whereas the moment a source of excitation has been removed fluorescence fades instantaneously, phosphorescence continues with progressively decreasing intensity. This may last anywhere from seconds to hours. Unlike Fluorescence, Phosphorescence indeed tends to feature exotic colours. This phenomenon is of particular relevance in Chamaeleon diamonds and certain intense yellow-green daylight phosphorescent fancy coloured diamonds.
Blue fluorescence visually enhances a diamond by giving it a brighter appearance in the eyes of most people (proven in blind tests according to the GIA) including diamond professionals. Fluorescence increases the apparent white saturation by causing internal refraction on a submicroscopic level somewhat akin to the finest silk in certain gemstones. Fluorescence has unfortunately received much undeserved negative attention in that it may sometimes reduce transparency by turning a diamond hazy or worse milky.
Previously Fluorescence, particularly in the American market, was highly regarded. However, with the rise of the Asian luxury consumer, the taste has slightly shifted, in parts perpetuated by misinformation.
Labs grade Fluorescence as none (absent) – faint/slight – medium – strong and very strong. Only when determined as strong or very strong should fluorescence be of concern and a diamond further checked under natural lighting conditions.
Far more relevant is the implication that fluorescence can lead to overgrading, overstating a stone’s true colour. Evidence shows that blue fluorescence acts complementary in yellow tinted ‘cape’ diamonds in 95% of cases imparting a more whitish face-up colour by partly cancelling out the yellow body colour. On the contrary yellow fluorescence may accentuate face-up colour. Hence the trade usually assigns price reductions in stones of I or higher colours ranging from 5% in I to 30% in D from medium to very strong, with differing discounts applied to various combinations. In colours J or lower prices, however, increase by as much as 10% whenever fluorescence enhances apparent colour and thus salability. Similarly, fluorescence is judged according to the effect it has in natural fancy coloured diamonds. This may be either positive or negative.
Unfortunately, Labs often assign strong to very strong fluorescent stones a lower colour, or in the case of fancy coloured diamonds, a lower intensity grade by default. This, however, is a questionable practice, given grading is done in a rather clinical environment far removed from reality.
Furthermore, Fluorescence in D to Z colour diamonds is often used as a marketing ploy to discredit fluorescent stones, further aggravating the pricing structure accordingly. The flip side is, that (non-investment level) stones with strong fluorescence can be undervalued and at the right price represent excellent value for money.