Fluorescence is the visible emission of light which occurs when a diamond is exposed to ultraviolet light.  It is a natural characteristic, observed in nearly one-third of all gem-grade diamonds, resulting from the presence of trace amounts of Nitrogen, a common substance in diamonds.

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.  In diamond certificates, fluorescence is usually classified as None, Faint, Medium, Strong or Very Strong.

Many people are falsely led to believe that fluorescence is a negative quality, when in fact it can have either a positive or negative effect on the colour and overall look of a diamond.  Diamonds with high colour gradings of D, E and F, exhibiting strong to very strong fluorescence, can look somewhat hazy or milky in direct sunlight.  However, those with relatively low colour gradings, around I or J, usually appear one or two grades better in colour due to the masking effect of blue fluorescence.

The effect of fluorescence on the value of a diamond depends entirely upon its strength and colour.  The stronger the fluorescence the greater the negative effect can be on the value of higher colour grades and conversely, the greater the positive effect on the value of lower colour grades.  The value of higher colour grades can be discounted by as much as 5% to 10% and the value of lower colour grades can be increased by a similar amount.  Weak or faint fluorescence tends to have little or no effect on value.