As for when you are proposing, the moment should be one the both of you will remember for the rest of your days, and one of the ways to ensure that is by getting the right diamond ring for the occasion.
In order to make sure that a diamond will have optimal fire and brilliance, you will need to make sure that it is fully free of milkiness, as well as of “clouds”. Luster is one of the effects of fluorescence, and it is not constant. If you get an L1 diamond, which is one with top luster, no amount of fluorescence would impede your diamond’s visual appeal.
Fluorescence is a property, which is rarely understood or explained, but at its basic, this is the visible reaction of a diamond when it is put in front of UV light. You do not have to be working at a rave to see how important this is – even the sun emits UV rays, and you do not want your 1 carat diamond engagement ring looking funny in daylight.