Alongside the four Cs, fluorescence is one of the most important things to look at when buying a diamond. If a diamond is fluorescent, it will emit a glow when exposed to a ultraviolet light source. (Sunbeds and nightclubs are the most common places you will find UV lights, though sunlight does contain UV light). This can make the diamond appear cloudy or oily.
Most fluorescence is blue, though yellow and orange fluorescence exists too. Orange fluorescence is a very rare thing, and can be prized by gem collectors!
Strictly speaking, fluorescence is a description, not a grading, but many people use the word “Grade” for convenience. There are five descriptions for fluorescence on a laboratory report: Nil, Slight or Faint, Medium, Strong and Intense. It is usually accompanied by a description of the colour, for example “Slight Blue” or “Medium Orange”. If there is no colour description, by convention it refers to blue fluorescence.
Fluorescence occurs in about one third of diamonds, and is caused by the presence of tiny amounts of nitrogen in the crystal. In general, it is better to have lower levels of it, though in some cases it can enhance the apparent colour of the diamond; this is most common in lower colours. The video below shows the difference between strong and zero fluorescence.