Fluorescence

Fluorescence is the visible emission of light which occurs when a diamond is exposed to long wave ultraviolet light. It is a natural characteristic observed in nearly 1/3 of all gem grade diamonds resulting from the presence of trace amounts of nitrogen a common impurity in diamonds.
Some people seek diamonds that produce this unique effect, while others definitely avoid it.

Why is this so?

The reason behind this is Fluorescence can have either a positive or a negative effect on the colour and overall look of a diamond when viewed in natural light.

Colourless diamonds with high colour gradings of D,E or F exhibiting medium to strong fluorescence can often look somewhat hazy or oily in direct sunlight. But not always.

However Diamonds with relatively low colour gradings around I or J particularly those with a yellowish tint usually appear one to two grades better in colour due to the masking effect of blue fluorescence. But not always.

The effect fluorescence has on a diamond is not put on a grading certificate only whether the diamond has fluorescence and of what strength therefore each diamond must be viewed to determine the effect.

The effect of fluorescence on the value of a colourless 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 grades. The value of higher grades can be discounted by as much as 5 to 10% and the value of lower grades can be increased by a similar amount. Weak or faint fluorescence tends to have little or no effect on value.