It can happen. Some diamonds glow soft blue, or another color, when exposed to ultra-violet (UV) light.
Chemicals in the earth where the diamond formed can leave trace elements in the crystalline structure. If certain impurities are present in your diamond they can be excited by ultra-violet light or x-rays, and re-emit visible light. We call this phenomenon fluorescence (there is also phosphorescence, which is another topic).
Yes. Fluorescent diamonds are traded at a discount, usually in proportion to the strength of fluorescence. DEF colors with fluorescence are discounted more severely than near-colorless diamonds and lower. Whatever the case, you will find that diamonds with medium and higher fluorescence nearly always trade at a discount.
The diamond is exposed to short-wave and long-wave ultra-violet light. When present, the strength and color of fluorescence is noted on the grading report. In the majority of cases fluorescence is blue, but it can take on (rare) other colors too.
This is absolutely the most important question to answer. In many or most cases fluorescence by itself does not hinder transparency or optical performance. But any fluorescent diamond must be considered on a case by case basis (read on).
Some people avoid it and some people love it. The question is this: Do you want your diamond to have a soft, eerie glow when you enter a nightclub? If you don't, seek a diamond with negligible fluorescence. If you like the idea, pursue medium-fluorescence or higher, as long as you're assured it's not milky, oily, hazy or cloudy (read on).
These terms are regularly tossed about to describe the deleterious effects of "fluorescence," with no further explanation. Although they are frequently used interchangeably, with no elaboration, each descriptor has a specific meaning:
Milky-Oily: In "overblue" cases these descriptions can occur in the same diamond, but in different lighting conditions.
Hazy-Cloudy: These elements can be completely independent of fluorescence, but become exaggerated by it.
In Strong and Very-Strong blue fluorescent diamonds which are oversaturated with boron the UV excitement causes a milky purple sheen through the diamond which interferes with transparency. These diamonds are nicknamed “overblues." Many or most came from the Premier mine decades ago when South Africa was the only diamond source. As they were regularly coming to market dealers began the practice of discounting fluorescence, along with a fearful attitude about any diamond with fluorescence, which became exaggerated. That attitude exists to this day among some jewelers. In response to this, GIA published a study in 1997 which debunked the idea that “overblues” are common, in fact they couldn’t even find enough for the study.
This is a combination of an overblue diamond, or close to it, and viewing in strong sunlight: When the blue/purple sheen saturating the diamond in UV daylight mixes with dispersion/fire it can have the appearance of oil shining on the surface of a puddle. Some diamonds which have a strong sheen show this when viewed in the sun, but you have to take them outside to see it and, again, they are so rare that most jewelry professionals have never seen one (except in antique and estate jewelery).
Microscopic pinpoints, persistent throughout a diamond, can affect transparency, even when unnoted at 10X: In grading, if a cluster of individual pinpoints is deemed insignificant at 10X they will be described as "not shown." If you need higher magnification to see them they'll be completely unmentioned. That's fine. But remember, the category is clarity, not optics. Optical performance is a different category and things unmentioned in the clarity section can hinder light transmission. In fact this happens with some regularity. Diamond trade professionals have a specific nickname for such diamonds, they call them "sleepy." When a hazy diamond has fluorescence those pinpoints become more noticeable when the fluorescence is reacting to UV, and can become detected. This has caused people to blame haziness on fluorescence (even medium fluorescence) but the fluorescence and haze are independent of each other. In fact, many slightly hazed, or slightly "sleepy," diamonds are never even detected by the average jeweler
In the style of hazy diamonds (which have persistent pinpoints) a diamond can have isolated but dense concentrations of pinpoints which reduce transparency. Again, these are completely independent of fluorescence, but those clouds are more easily detected when fluorescence is reacting to UV and illuminates them.
Crafted by Infinity provided us with these rare images of rough crystals which were rejected due to over-strong fluorescence.
There is a segment of the consumer market that loves blue fluorescence and seeks out such diamonds. That is fine as long as the shopper is fully aware of the pros and cons when making that buying decision. Fluorescence has historically been treated with censure and can impact the ability to sell or trade a diamond. For this reason it's critical that any diamond with fluorescence be supported by strong future upgrade or buyback policies.