If a diamond has a color
grade of J to M, a moderate amount of fluorescence will actually
make a diamond more attractive to most people. Slight to moderate
or even strong blue fluorescence in a stone with these color
grades actually helps cancel some of the yellow and makes it
looks whiter.
However, for a diamond with very high color (such as a D to F
grade), fluorescence is thought to interfere with the flow of
light and make the diamond appear a little oily or murky. This
will not be true for most diamonds, but it is thought to be so
and you can buy if for less. We still feel a little hesitant
to recommend D to F color diamonds with strong fluorescence,
despite the recent research. This is just because of our many
years of training that we should avoid them. For this reason,
we still might recommend None or Slight flourescence in colorless
diamonds (D to F color grades).
In grades inbetween
those above (G, H and I), it might be better to stay away from
Strong fluorescence for safety's sake, while moderate fluorescence
might actually improve the color appearance