Facts of the Matter
Diamonds made from the stuff of life
THE ALLURE of diamond makes it the most highly revered among gemstones. It has many qualities that make it such a desirable and beloved, not the least of which is its rarity.
They are the hardest, the most brilliant, and the most imperishable of all precious stones. The word "diamond" comes from the Greek word adamas, meaning "unconquerable."
Their clarity, brilliance and fire are rivaled by no other gem, yet they are made from carbon, one of the most remarkable chemicals in the periodic table. It is the blackness in a lump of charcoal, and is also the chemical element responsible for life itself.
Diamond is carbon in its most concentrated form, and except for trace impurities like boron and nitrogen, which give it tints of pink and yellow, its purest form as well.
Diamonds have been a source of fascination for centuries. From myths about valleys of diamonds protected by snakes, to the production of millions of carats in rough diamonds each year, the history of diamonds, dating back some 3,000 years to India, is one of mystical power, beauty and commercial resourcefulness.
Only a few diamond deposits were known until the 20th century, when scientific understanding and technology extended diamond exploration and mining around the globe.
The brilliance and fire of diamonds are related to their refractive index.
Today, diamonds are mined in about 25 countries, on every continent but Europe and Antarctica. Surprisingly, more than one-third of the world's diamonds come from Australia.
Diamonds are formed deep within the Earth, in the mantle between 60 and 120 miles below the surface where temperatures are 1,700 to 2,400 degrees Fahrenheit and the pressure is 45,000 to 60,000 atmospheres..
The youngest known diamonds were formed 1 billion years ago and many are more than 3 billion years old.
Volcanic eruptions carry diamonds to the surface via a magma conduit known as a kimberlite pipe. Most diamonds occur at the surface in an ordinary looking volcanic rock known as kimberlite.
The kimberlite magmas that carry diamonds to the surface are much younger than the diamonds they carry. The preexisting diamonds simply ride the kimberlite magma like an elevator.
Only 20 percent of diamonds, or fewer, are of gem quality. The rest fall into two other categories known as near-gem quality and industrial quality.
Near-gem quality diamonds can be used as either gemstones or for industrial purposes, depending on the individual stone
The distinctive fire and brilliance that give diamonds their extraordinary appeal is the strong chemical bonds between carbon atoms.
Carbon atoms have four bonding sites, and have an uncanny facility to form bonds with other carbon atoms by sharing electrons at those bonding sites.
This forms the most perfect of covalent bonds, wherein the electrons are shared equally. In most covalently bonded molecules, such as water the probability of finding an electron is greater near the oxygen atom than either of the two hydrogen atoms. In a covalent carbon-carbon bond, electrons are equally distributed among the bonded atoms.
In the perfect covalent bonds of the diamond lattice, the carbon atoms form a three-dimensional, interlocking lattice of tetrahedral-shaped diamond structures.
These bonds are likewise responsible for the primary optical properties that give diamonds their distinctive fire and brilliance.
A transparent surface can either reflect or refract light that strikes it and the interaction of the atomic bonds with the light determines the nature of the interaction.
The relevant aspect of light is its oscillating electromagnetic field. The frequency of oscillation is what determines the color of the light. When light enters a solid, its progress is slowed because it has to "jiggle" electrons in order to pass through.
Because the electrons in the diamond lattice are tightly bound to the carbon atoms, they respond sluggishly, which slows down the speed of light passing through it.
THE BRILLIANCE and fire of diamond are related to its refractive index, which is the highest of any transparent material.
The refractive index (RI) is the speed of light in a vacuum compared to the speed in a medium. Diamond has an RI of 2.54, which means that the speed of light in diamond is only forty-one percent of that in air.
Diamond's high RI makes it the most reflective of all transparent gems. A well-polished diamond's outer surface reflects 17 percent of the light that strikes perpendicular to it. By comparison, well polished glass reflects only four percent.
This gives diamonds a very high luster when compared to other gem materials.
Light that is not reflected enters the diamond. That which enters perpendicular to the surface is only slowed, but light that enters at any other angle is bent toward the perpendicular. The higher the RI, the more it is slowed and the more the light is bent.
The RI is not the same for all colors of light, a process known as dispersion. Even ordinary glass has dispersion, which is why a prism separates light into a spectrum.
The listed RI of a transparent solid refers to yellow light, which is in the middle range of visible light.
Dispersion is also the principle behind rainbows, as tiny water droplets refract and disperse sunlight.
Diamond has the highest dispersion of any transparent medium. Although its dispersion is only a scant one-half percent greater than glass, it is enough to give the diamond its fire.
If light enters the solid at a steep enough angle, it is bent so much that it cannot emerge from the opposite side. It is reflected internally until it strikes an inside surface at an angle that allows it to escape.
Gem diamonds are cut to keep the light bouncing around inside as long as possible. By cutting the facets at just the right angles, the light will reflect off many surfaces, trapped inside by internal reflection.
The farther the light travels, the greater the separation of colors and the greater the fire.
Fire is not the same as dispersion, although it may seem so. Fire is the appearance of colored flashes, more correctly called chromatic flares.
Light rays of different colors, coming in at the same point and direction, can take different paths through a polished diamond.
Fire is influenced by a diamond's proportions in four ways: The angle that light enters a diamond; the angle that light exits a diamond; the number of reflections the light has inside the diamond, and the number of times that light rays spread across facet junctions
Different illumination conditions and surroundings (the "viewing panorama") can enhance or diminish the appearance of fire so that different individuals do not see the same fire in a given diamond, adding to the mystery and allure of the gem.
The combination of diamond's physical properties of hardness, refractive index, dispersion and range of colors make it the most highly sought and most expensive gem on the market.
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to firstname.lastname@example.org