Fluorescence
Diamond fluorescence refers to the intensity of a coloured glow (usually blue) that is visible when a diamond is exposed to UV light. In some instances, diamonds with strong fluorescence may look milky and lifeless in daylight, but in most cases, the appearance of a diamond with strong fluorescence is rarely affected. Diamonds with none or a faint fluorescence are rarer than those with a strong or very strong fluorescence.
The degree of fluorescence can vary as around one-third of all diamonds show some fluorescence, with 95% exuding a blue colour – though green and yellow fluorescence diamonds can also be found, though much rarer than those with a blue hue.