They are so fucking cool. If I had the money, I'd have a room that's just these gems+blacklights. There are some that change color based on the wavelength of UV light they're exposed to. Some glow a different color under UV than they do under normal light. Some are both fluorescent and phosphorescent, meaning they light up in response to UV, but then they can maintain their glow temporarily. Some change color based on the angle you view them at. They're so fucking cool.