Why would some diamonds look a different color under a black light?
The color you are seeing under untraviolet light (a “black light”) is called “fluorescence”. In diamonds, fluorescence is caused by the presence of small amounts of elements other than carbon in the diamond crystal structure. (Pure diamond would be made of only carbon atoms.) Common impurities that cause fluorescence include nitrogen, boron, and argon.) About a third of all gem grade diamonds have visible fluorescence under ultraviolet light, and by far the most common fluorescent color is blue. Nevertheless, different concentrations and compositions of impurities can give rise to different flourescent colors. (See source for some images of different fluorescent colors of uncut diamonds.) The fact that the diamonds in your ring fluoresce differently simply means they have different trace-element compositions. The composition of a natural diamond is determined by the chemical conditions present when the diamond formed within the Earth’s mantle (and possibly modified by any chemical, the
