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:

• In the top colours (D–H), strong blue fluorescence can occasionally lend a faint milky or oily look in UV-rich daylight, lowering the price.
• In tinted stones (I–M), a clear blue fluorescence can offset a yellowish tint and give the impression of colourlessness in daylight — a genuine, if subtle, favour to the stone.