When diamonds are exposed to UV ray, some visible lights will be emitted. About 35% of the diamonds will have fluorescence reaction. Blue is the most common color, and other possible colors are yellow, orange and white.
Diamonds with strong or very strong fluorescence reaction look somewhat oily or milly sometimes, which weakens diamond's fire, therefore the price will be set lower.
The level of fluorescence: none, faint, medium, strong and very strong.