Diamond fluorescence
Many diamonds glow when ultraviolet light falls on them, most often blue. Labs report how strongly: None, Faint, Medium, Strong or Very Strong. Switch on the lamp and see what each means, then what GIA's research found about how fluorescence affects a diamond's look in daylight.
The five strengths under the lamp
The same diamond at each strength, in a dark room under long-wave UV. Tap one to load it above.
What GIA found
GIA, Gems & Gemology, Winter 1997: four sets of very similar round brilliants, colourless to faint yellow, chosen for different strengths of blue fluorescence and viewed by trained graders, trade professionals and average observers in several positions and lights.
Strongly blue fluorescent diamonds were seen as having a better colour appearance viewed table-up.
Viewed table-down, as colour is graded, no trend was seen.
For the observers representing the jewellery-buying public, no systematic effect of fluorescence was found.
Most observers saw no relationship between fluorescence and transparency.
GIA notes that excessive fluorescence can make a stone look cloudy or oily, which can lower its value. Try "Very Strong" with "Show the rare hazy case".
Strong blue fluorescence can make a light yellow diamond look closer to colourless in sunlight, GIA notes: daylight carries ultraviolet.
Sources
GIA, Diamond quality factors: many diamonds emit visible light (fluorescence) under UV; blue is the most common fluorescent colour in gem-quality diamonds; strong blue fluorescence can make a light yellow diamond look closer to colourless in sunlight; excessive fluorescence can make a stone look cloudy or oily. T. M. Moses, I. M. Reinitz, M. L. Johnson, J. M. King and J. E. Shigley, A contribution to understanding the effect of blue fluorescence on the appearance of diamonds, Gems & Gemology 33/4 (1997): the study design and findings summarised above. Report terms None, Faint, Medium, Strong, Very Strong: as used on GIA reports.
Images: ray-traced round brilliants (JewelLoop renderer). The glow, its strength and the haze are illustrations: real fluorescence varies with the stone, the lamp and the distance, and a screen cannot reproduce ultraviolet.