The Science Behind the Sparkle
What lies behind the dazzling dance of light in
a diamond? Read on.
by Robert Hazen
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
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.
In the 1700s, gold miners in Brazil tossed aside a fortune
in unrecognized raw diamonds.
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 for 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.
crystalline structure, here shown modeled against a diamond, slows light down
like no other substance on Earth.
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.
Light inside a
diamond travels at less than half its speed in a vacuum.
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 pokes along inside a diamond at less than 80,000 miles per
second. That's more than 100,000 miles per second slower than in air.
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.
Light entering a diamond ricochets around until
it can find a way out, broken into its constituent wavelengths.
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
Robert M. Hazen is a research scientist at the Carnegie Institution of
Washington's Geophysical Laboratory and Robinson Professor of Earth Sciences at
George Mason University. This article was excerpted with permission from
Hazen's book, The Diamond Makers: A Compelling Drama of Scientific
Discovery, published by Cambridge University Press in 1999.
Photos: (1-4) ©BBC; (5) Reprinted with the permission of Cambridge University Press.
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