Today, most people admire diamonds for two exceptional attributes: their hardness and their brilliance. Scholars knew of diamonds' unrivaled hardness since antiquity, but its unique optical properties went unrecognized until comparatively recently. The natural diamonds that were hoarded by potentates of old have little of the visual drama that we associate with today's faceted gems. Deeply colored rubies, emeralds, and sapphires were far more prized as adornments. Owners accumulated the seemingly indestructible diamond pebbles, as found in their unpolished natural state, as talismans against defeat and symbols of their own "manly" virtues, without ever seeing diamonds as objects of beauty.
When unearthed, most diamonds appear roughly rounded with perhaps a hint of regular crystal form. Many are colorless, but most are pale shades of yellow; red, orange, green, blue, brown, and even black diamonds are also found. Raw, uncut stones lack the exuberance of jewelry-store gems and can appear quite ordinary; Brazilian gold miners of the 18th century cast aside a fortune in unrecognized diamonds while panning for the precious metal. Diamond's familiar ornamental role represents a relatively recent development—a consequence in part of scientists' growing understanding of the nature of light.
A few scientific ideas have become part of our folklore. The equation E=mc2 is one—a cultural icon as much as it is a statement of the equivalence of mass and energy. Another of these commonplace science snippets tells us that the speed of light is constant. A T-shirt slogan popular in physics departments proclaims "186,000 miles per second: It's not just a good idea, it's the law!" But as with many other legal systems, there's some fine print most people ignore. You have to add the rather mundane words, "in a vacuum." When light travels through matter—air, water, glass, or diamond, for example—it travels slower than 186,000 miles per second. The actual explanation, having to do with the way light interacts with the electrons present in every atom, is somewhat complex, but you can visualize this slow-down by thinking of light rays having to make little detours every time an electron gets in the way.
Other natural gemstones disperse white light to some degree, but none comes close to diamond's ability to reveal the rainbow.
Most clear and colorless objects retard light only a modest amount. The air we breathe has only a trifling billion-trillion atoms per cubic inch. Space between atoms are much greater than the size of the atoms themselves, so air reduces light speed by just a few hundred miles per second—not enough to notice under most circumstances. In water and ice, which have thousands of times more atoms per cubic inch than air, light travels about 140,000 miles per second—30 percent slower than in a vacuum. Window glass drops light speed to 120,000 miles per second, similar to the travel time through most common minerals, whereas lead-containing decorative glass, the kind used in chandeliers and cut glass, slows light even more, to about 100,000 miles per second (lead has lots of electrons that get in the way).
Diamonds put the brakes on light like no other known colorless substance. Diamond is crammed with electrons—no substance you have ever seen has atoms more densely packed—so light pokes along at less than 80,000 miles per second. That's more than 100,000 miles per second slower than in air.
Most people never have reason to notice the variable speed of light, but you experience one of its consequences every day. Each time light passes from one clear substance into another with a different light speed, the light rays have a tendency to bend. You've probably noticed the distortion of people and objects in a swimming pool, which occurs when light waves have to change direction as they pick up speed coming out of the water. Ripples on the pool's surface compound the angular distortion. If you wear glasses or contact lenses, which "correct" the way light bends into your eyes, you take advantage of this useful optical phenomenon.
Light does not always bend when passing between different materials. If light rays strike a clear substance head on or at a modest angle—like the path of light coming through your window—most of the rays will travel straight through without bending. You can look down from a boat at the nearly undistorted bottom of a calm, clear lake or pond because sunlight enters the water from overhead and then comes back through the transparent water almost vertically to your eyes. But try as you might, you can't see the bottom of even the clearest lake standing on the shore, because you are at too low an angle to the water. Almost all of the light reaching your eyes has been reflected off the water's surface. That's why you can see the beautiful mirrored reflection of trees on the opposite shore of a glassy lake early in the morning.
Diamond plays this reflecting trick better than any other colorless substance. Light enters a faceted gemstone from all sides, but it may bounce back and forth several times inside before it finds a clean, straight shot out. All this changing direction accomplishes something very dramatic, because so-called white light actually contains all of the rainbow's colors. Each color—red, orange, yellow, green, blue, and violet—bends and reflects inside the diamond slightly differently. The farther the light travels, the more the colors separate, or "disperse." Bounce light inside a diamond just two or three times and the colors disperse spectacularly. Diamond-like substitutes, including "cubic zirconia," a crystalline compound synthesized from the elements ziconium and oxygen, attempt to mimic this light-dispersing property, though they fall short of diamond's brilliance and unrivaled hardness.
If you look closely at a faceted diamond, you can see that it soaks up white light and breaks it apart like a prism, dispersing it into a rainbow of colors. Diamonds sparkle and dance with colored light; each of its dozens of facets produces its own dazzling display. Other natural gemstones disperse white light to some degree, but none comes close to diamond's ability to reveal the rainbow.