The Importance of Fluorescence
When certain gemstones are exposed to invisible ultraviolet rays such as the sun and fluorescent light, there is a visible light known as fluorescence that is reflected to the human eye. The most common colour of fluorescence seen is blue, however either colours may be noticeable. The difference in variations This is a characteristic that is noted on diamond grading certificates alongside the 4cs (colour, clarity, cut and carat weight).
The image guide from GIA indicates how difficult it is to find the fluorescence emitted.