Under ultraviolet irradiation, about 35% of gem-grade diamonds will fluoresce. Most gem-grade diamonds that emit visible fluorescence are blue, and a few emit yellow fluorescence. The classification of diamond fluorescence by different appraisal institutions is slightly different. For example, the diamond appraisal report of GIA Gemological Institute of America, which is well-known in the world, divides the grades describing the fluorescence of diamonds into five grades: none (none), dim (weak), medium (medium), strong (strong) and very strong (very strong). Another example is the diamond appraisal report of the Belgian Diamond High Council in HRD Antwerp, which is also well-known internationally. There are four grades to describe the fluorescence of diamonds: none (none), thin (weak), medium (medium) and strong (strong).