Fluorescence

Fluorescence is a naturally occurring phenomenon: certain minerals and gems glow when struck by ultraviolet light. Around a third of all diamonds fluoresce to some degree, most often a soft blue visible under UV — invisible in ordinary daylight.

The degrees of fluorescence

Graders record fluorescence by strength, from None through Faint, Medium, Strong and Very Strong. Blue is by far the most common colour, though others occur. It is one of the observations made during colour grading, because it reveals a stone’s type and whether colour has been treated.

How it affects a diamond

The effect cuts both ways:

• Among the finest colours (D–H), a strong blue reaction occasionally softens a stone under daylight rich in ultraviolet, leaving it faintly hazy or oily to the eye. Buyers tend to pay less for it.
• In faintly tinted stones (I–M), the same blue glow works the other way. By cooling the warmth of a yellowish body colour, it can make such a diamond read whiter than its grade suggests under natural light — a quiet, if modest, kindness to the stone.