We get questions here at Moriarty's Gem Art all the time. Either people coming into the store or online through our many websites. This question is one we have gotten many times, and the latest one was actually though our Facebook page. My brother Michael (the one doing all the setting work), actually said we should do a blog post about it, since it's asked all the time. I agreed. So what is the question?
Why do some diamonds fluoresce under UV lamps and others don't?
Some diamonds fluoresce under UV radiation/light, including sunlight, because of defects or elemental additions to the subatomie/submicroscopic structure of the diamond. Some elemental additions will cancel out fluorescence. Fluorescence is not necessarily a "defect" or negative aspect of a diamond, but an identifying characteristic. However, some with a strong blue fluorescence can have an oily or cloudy appearance when viewed in natural bright sunlight. That's why it is important to view the diamond before purchasing. Hope this answers your question, and if you would like some additional information you can visit GIA, the best source for Diamond information.
Make sure to visit us online, or if you are in northwest Indiana, visit our store in Crown Point, IN - Moriarty's Gem Art.