Some diamonds emit a soft colored glow when subjected to ultraviolet light so called a "black light".

Is fluorescence good, bad or just ugly?

It depends. In some cultures (such as Vietnamese) strong fluorescence is desirable. But most people would agree that diamond fluorescence is a nuisance to be avoided.

Colorless (D-E-F) or near colorless grades (G-H-I-J) are lower in price when they exhibit fluorescence, however warm toned grades (K-L-M) are higher in price.