If a diamond engagement ring fluoresces “faint” or “weak” the effect on the diamond color grade or value is negligible. If it is graded as “more intense” or “very strong” fluorescence, it will affect the value negatively.
If a diamond engagement ring has “medium” fluorescence, it can change the body color grade ½ to 1 color grade different than its actual color; if “strong” by 1-2 color grades and if “very strong” by 3-4 color grades. If the stone fluoresces whiter than it actually is under UV lighting, you may be paying too much. If it fluoresces temporarily lower in color you may be paying less than what it is worth. As it relates to fluorescence, if you are paying less, you are getting a higher color graded vintage diamond ring than it actually shows. Using a professional gemologist to grade your diamond engagement ring with UV free lighting will give you the true grade of the diamond which determines its actual value.
If you are considering a one carat diamond engagement ring with “very strong” fluorescence and is color graded at E, you can expect the actual color to be H-I. That would be three color grades difference and will cost considerably different. A non-fluorescent internally flawless clarity and E color would cost you $14,700 per carat for a one carat as of September 2011 and a non-fluorescent internally flawless clarity and I color would cost $6300 per carat for a one carat. If your stone is not priced accordingly taking fluorescence into consideration, find another diamond.
If a diamond is priced right, there is nothing wrong with a fluorescent diamond.