When light meets an object, it may pass through the object with no effect, be absorbed by the object, or be reflected off the object. Light may also pass directly through certain objects, changing direction when it passes from one medium to another in the process. The name for this phenomenon is refraction.

Refraction occurs when light leaves one transparent substance, such as air, and enters a second one through which it travels at a different speed. Because air, water, glass, and diamonds possess different densities and atomic structure, light refracts when it passes from one of these media to another. When light passes out again, it resumes the speed it was traveling before it entered the new medium.

The atoms in a diamond are tightly packed with electrons -- so tightly that light travels at less than half the speed through the crystallized carbon as it does through air. In fact, no other transparent material slows light more than diamond. Jewelers use this unique property to temporarily trap light. They cut and polish diamond in its rough, natural state to produce a gem with smooth sides called facets. Light enters from all sides, and once inside, it bounces off several facets before leaving. The farther the light travels before leaving, the more it separates into the rainbow of colors that make it up. A diamond's dazzling sparkle, then, results from light's bouncing travel.