Blue Fluorescence
if strong or very strong, may alter the perceived color of a diamond in a negative or positive way. For example, stones in the colorless/near colorless ranges (D-H) may appear milky or oily, detracting from their appearance. On the other hand, diamonds in the lower, more yellow color ranges (I and lower) may appear to have less yellow color due to the fluorescence, adding positively to their appearance. Thus, the trade will slightly discount prices of diamonds in the former category, while sometimes adding a very slight premium to those in the latter.
The presence and color (most frequently blue, but can be any color) of fluorescence and its intensity (none/inert, faint, medium, strong, very strong) are indicated on all GIA and AGS grading reports.
Irradiation
of diamonds using a nuclear reactor or linear accelerator, sometimes in combination with heating, is used to produce a variety of different fancy colored stones.
High Pressure/High Temperature (HPHT) treatment
is a process developed by General Electric whereby type IIa diamonds of low color (N-O) or even fancy brown color, can be converted to colorless/near colorless (D-H) forms by an annealing process involving high pressure and temperature. There is some concern that these color-enhanced stones may come to market undetected, but new research has shown that they can be largely detected using expensive and sophisticated equipment. Similarly, the HPHT technique has also been applied to type Ia brown diamonds by several companies, yielding fancy yellowish green and greenish yellow colors. Pink and blue colors have also been produced by the same technique.
The Color Grading Scale
 ranges from totally colorless to fancy yellow. The differences between one grade and another are very subtle, as can be seen by the number of grades within any one category.
