Fluorescence Effect on Color (7.42)
Negative Effects: The term blue-white diamond's origin was strong blue fluorescence combined with high color. The FTC banned the use of the term in marketing in 1938 since it was eventually realized that it did not add beauty to the diamond but in some cases compromised it.
Faint Blue Fluorescence will not be noticed under sunlight or normal light conditions, if accurately described, but can be detected with a long wave ultraviolet light source. The negative effect on value is slight.
Medium Blue Fluorescence will almost always be observed in direct sunlight and many times under artificial light. The negative effect on value is moderate to substantial.
Strong Blue Fluorescence may show a subtle haziness in artificial light but will always show obvious haziness in direct sunlight. The negative effect on value is substantial.
Very Strong Blue Fluorescence is often valued as an imperfect diamond since it always displays haziness under any light conditions. The negative effect on value is very substantial.
The above images are courtesy of: https://filigreejewelers.com/products/flush-set-old-mine-cushion-cut-diamond-ring-in-14k-yellow-gold?srsltid=AfmBOorUSEXV2ZquowJQVXKcxU5S_TRSOjceSRP5sZju6t5-bt3l2dRR
Positive Effects: In faint yellow diamonds the medium to strong blue fluorescence can hide the tint in a face up position (as in a mounting). It can potentially make a K-L color look like a G-H color in a mounting so be sure to check the diamond with your ultraviolet light source before a purchase (unless it is confirmed with a GIA report).