Why Is It Blue? Light interacts with an impurity in the diamond to produce the unusual color. As the diamond grew, a few atoms of the element boron entered the crystal structure. The addition of just one boron atom for every million carbon atoms is enough to cause the deep blue color. Blue diamonds are extemely rare. Only about one in 100,000 diamonds is strongly colored, and blue is one of the rarest colors.
Another Light on the Hope
After ultraviolet light is shone on the diamond and removed, the diamond glows vivid red-orange. This phenomenon, called phosphorescence, is only visible in a very dark room. Although some other diamonds phosphoresce, the red-orange color of the Hope is highly unusual. It is caused by trace impurities in the diamond.