Fluorescence is one of the more dazzling properties of minerals. I mean, how can you not be wowed by this?
How does that even work?
First, you need three ingredients: a mineral, darkness, and an ultraviolet light. A moment to go over that last one.
Light comes in different wavelengths, ranging from infrared to ultraviolet (or UV). Fluorescent minerals "glow" under the high-frequency, short wavelength ultraviolet light. But even within ultraviolet there's a range.
The kind you probably have at home—the kind you can buy in Spencer's or Halloween stores—is a "long wave" black light, with emits ultraviolet light at a wavelength of about 380-385 nanometers.
The other kind is, predictably enough, a "short wave" black light. They aren't sold in regular retail outfits; you need to go to an online specialist or a rock and mineral shop to find one. The wavelength on a short wave back light can be as little as 254 nanometers.
What does this mean for minerals? It means that the fluorescence of a mineral will change depending on the black light you use. Some minerals will only fluoresce within a very specific range, others will change colors.
But anyway: you have your rocks, you have your black light, and you've turned out the other lights. Enjoy the fireworks for a moment.
But where do those colors come from?
When such a high frequency light hits the atoms inside, they get excited. And then, soon after, un-excited. It's the movement from excitement to boredom (for lack of better term) that causes the colors we see (in the movement "down," so to speak, the atoms will emit a photon—light, basically).
Not every specimen of every possibly fluorescent mineral will necessarily fluoresce. Plenty of fluorite fluoresces, but plenty of it doesn't. For that reason, all you rockhounds, it's best to buy your fluorescent rocks in person.