That Inner Glow

Rocks may be the ultimate in boring, beating even drying paint and growing grass. They're hard and gray and they just sit there, doing nothing. Right? Not these rocks. These rocks have a hidden talent. They can fluoresce in breathtaking Day-Glo colors.

How do they do it? Each rock contains a sprinkling of atoms that respond to ultraviolet light, high-energy light that we cannot see. The blue rock, for example, is fluorite, a common mineral. Pure fluorite remains dull under ultraviolet light. But if just one atom in 10,000 in the fluorite is the element europium, the rock starts to glow. When ultraviolet light shines on the rock, the electrons in the europium grab the energy in the light and rocket to a higher level in the atom, then tumble back down to lower levels. Because they rocket up in one step but dribble down in several steps, the light they release is at a lower energy. They take in invisible ultraviolet light but they return it as a blue glow we can see.

This talent is not unique to rocks. Nearly all laundry detergents include a fluorescent dye that glows blue when exposed to sunlight, in order to make dingy whites shine brighter. Now that is boring.