Fluorescence is a natural characteristic, observed in nearly one-third of all gem-grade diamonds. Diamonds with fluorescence will glow when exposed to ultraviolet light. This reaction is mostly visible using a UV lamp, but in some stones the reaction is so strong that it can be seen in sunlight.
On diamond certificates, fluorescence is usually classified as None, Faint, Medium, Strong or Very Strong followed by the fluorescing colour. The most common fluorescence colour observed in diamonds is blue, although other colours are possible, such as yellow, white, green and very rarely orange and red.
Diamonds with high colour gradings such as D, E and F, exhibiting strong to very strong fluorescence, can look somewhat hazy or milky in direct sunlight.
It is a common misconception that fluorescence is always a negative quality – in some cases it can actually have a positive effect by appearing to improve the colour of a diamond. Those with relatively low colour gradings, around J, K or L, usually appear one or two grades better in colour due to the blue fluorescence balancing out yellow tones within the stone.
The effect of fluorescence on the value of a diamond depends entirely upon its strength. The stronger the fluorescence, the greater the negative effect can be on the value of higher colour grades. The value of stones with strong fluorescence can be discounted by as much as 5% to 10%. Conversely, the value of lower colour grades can be slightly increased. As weak or faint fluorescence has minimal effect on the appearance of the diamond, it tends to have minimal effect on the value.