Diamond fluorescence is one of the least understood properties of diamonds, largely because the attitude of dealers and collectors to fluorescence has changed over the years. The bottom line, of course, is how it affects the value of a diamond. Unfortunately, if you ask any ten experts this question, you'll probably get ten different answers from them. There are very few topics which provoke more controversy among diamond experts.

Fluorescence is the quality that some stones have of glowing when subjected to ultraviolet light (UV or "black light"). This can be anything from a very faint glow to a very strong effect. There are a couple of practical effects of fluorescence. First, it tends to improve the appearance of diamond color. So if an H color diamond, for instance, has medium blue fluorescence, its color may appear a couple of grades higher. The other effect, however, is not as attractive. The more fluorescent a stone is, the more likely it is to have a slight haziness which will affect the brilliance of the diamond.

Many jewelers will tell you that fluorescence is a negative quality in a stone, but there are many people who actually like fluorescent stone. And that's pretty sensible, because the fluorescence detracts nothing form the fire and flash of the stone while it makes them look a little whiter than they actually are. In any case, this is usually a very subtle effect and you probably won't even notice that your diamond is fluorescent until you happen to see it under ultraviolet light.

Fluorescence is almost always blue, but there are diamonds which fluoresce in other colors. One of the most famous examples is the celebrated Hope Diamond. This dark blue diamond actually fluoresces blood red under ultraviolet light.

A number of recent studies have tended to contradict the instinct of most jewelers by saying that the fluorescence has very little impact on a stone's appearance. Probably this prejudice of dealers stems from the 1950s, when diamonds with fluorescence were highly favored and sold at a premium as "blue-white" diamonds. This was largely a marketing scam and after black lights became common in the 1970s, the color was exposed as fluorescence and blue-white diamonds weren't much heard of after that.

However, when you are buying a diamond, you should check it for fluorescence. This is probably not going to be a major factor in the appeal of the stone unless it is very strong, but even then it is not necessarily something which detracts from the stone's appearance.

If you're considering buying a stone with a color grade somewhere between J and M, then a slight to moderate blue fluorescence will make the stone more attractive.