American Scientist

If you look at a collection of solid, pure chemical elements, they are mostly a pretty monotonous bunch. But carbon is the superlative standout. A lump of pure carbon atoms can range from black noncrystalline carbon to slippery crystalline graphite to crystal clear diamond, which is the only room temperature form of any solid element that can be transparent and “clear as crystal.”

What differences there are between these different carbon forms! Noncrystalline carbon is glassy; it has no geometric order between its individual carbon atoms. Graphite, on the other hand, is perfectly crystalline. It is made of stacked sheets of carbon atoms arranged in hexagons linked together like chicken wire. The sheets are only loosely bound to each other, and graphite, the shedding dog of carbon, constantly sloughs tiny dandruff flakes when it is handled. You can tell when a person handles a lot of graphite because their hands are dirty, covered by black, slippery residue. These graphite flakes get on their clothes and anything that they touch.

There is no greater contrast to soft, sooty graphite than diamond, the hardest known material, which is clear and totally transparent in its pure form. You can use slippery graphite dust to ease your key in and out of a troublesome lock, but you would never try this with diamond dust. Its shards are gritty, used to abrade the hardest known materials. Diamond is extraordinary in many other ways; for example, it is also the best conductor of heat, has an extremely high melting point, and is a good electrical insulator.

Both graphite and diamond are forms of pure crystalline carbon, but graphite is composed of sheets, whereas diamond is composed of carbon atoms in a three-dimensional, strongly bonded structure. The unusual properties of diamond are related to its regular, repeating pattern of atoms: Each carbon atom forms strong bonds with its four closest neighbors, with the angle between bonds being slightly more than 109 degrees. This diamond structure has cubic symmetry, and mined diamonds often display octahedral shapes—that is, the form of two pyramids joined at their bases. Out of the millions of compounds that carbon makes, diamond is the only one that features these bonds, which form a remarkably strong crystalline structure. These bonds are set in stone, so to speak, and they are responsible for the superlative properties of diamonds.

Out of the millions of compounds that carbon makes, diamond is the only one that features cubically symmetrical bonds, which form a remarkably strong crystalline structure.

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Although the cliché “Diamonds are forever” is not always true, still, in most imaginable scenarios and conditions, diamonds can last nearly forever. Some diamonds in meteorites are actually older than the Earth and the Sun. In the right conditions, however, diamonds can turn into graphite or even combust into carbon dioxide.

Special conditions are necessary to create diamond: On our planet, the needed pressure and temperature are found only far beneath Earth’s surface. Most diamonds were formed in Earth’s upper mantle, about 100 kilometers below the surface of old continents. Diamonds are nearly pure carbon, but they do carry small amounts of other elements and embedded solids. The gems are classified into types based on whether they are pure carbon, contain nitrogen atoms, or contain boron atoms (other impurities are possible, but the classification system considers only those two elements). About 99 percent of diamonds that are mined are classified as Type I, meaning they contain nitrogen. The nitrogen content gives the gems distinctive properties, such as a slightly yellow or grayish hue. Only about 1 percent of small diamonds have the color, clarity, and low enough impurities to be classified as precious Type IIa stones.

Over the past few years, a most interesting class of giant diamonds has been recognized, almost all of which are classified as IIa. These monsters are clear, but they do contain tiny inclusions whose properties indicate that these best-of-the-best diamonds have quite a different history than common diamonds. They have superdeep origins—that is, they formed several times deeper than common diamonds. Superdeep diamonds that are large and have extraordinary clarity, color, and other properties are called CLIPPIR (Cullinan-like, large, inclusion-poor, pure, irregular, and resorbed) diamonds. The most famous CLIPPIR diamond is the Cullinan from South Africa, which at 3,106 carats (0.61 kilograms!) is the largest gem diamond ever found. It was cut into nine principal stones, all of which are part of the British Crown Jewels.

Not all superdeep diamonds are large; most are small, but they are all scientifically quite important. They are our deepest samples of solid matter from inside our planet and thus contain direct knowledge about the deep interior of our planet that otherwise would forever lie beyond our direct reach. We can send spacecraft to the edge of the Solar System and even beyond, but it seems unlikely that we will ever build a device to retrieve samples from even the top of Earth’s lower mantle, 660 kilometers beneath the surface, where the temperature is 1,600 degrees Celsius and the pressure is 24 gigapascals.

Superdeep diamonds are magic vehicles that transport tiny inclusions of deep materials to the surface, like small stowaways, protected by the incredible robustness of diamond. They provide a unique means to explore Earth’s deep interior, which includes the mantle transition zone (MTZ) between 410 kilometers and 660 kilometers below the planet’s surface.

Some superdeep diamonds contain inclusions such as ringwoodite, a mineral created under high pressure that can contain hydrogen and oxygen bound as a hydroxide inside the mineral’s crystal structure. Ringwoodite was first discovered in 1969 as a purple silicate inside a meteorite that had been “shocked” in space by a collision with another solid object. In Earth’s materials, ringwoodite has been found in nature only as small inclusions in superdeep diamonds. Because ringwoodite can include hydrogen and oxygen, the building blocks of water, its discovery in the MTZ could contain information about the origins of Earth’s oceans.

The differences between deep and shallow diamonds provide fundamental insight into Earth’s interior processes and history. The presence of metallic iron in superdeep diamonds suggests that the carbon that formed these gems may not have been carried down from the surface, as with common diamonds, but up from below. The carbon in these superdeep diamonds may have previously been in iron metal, which exists in the lower mantle and is also the primary material in Earth’s core.

Finding Diamond Pipes

Diamonds form deep, but they travel to the surface in a most interesting way. In the Earth’s past, there have been unusual types of volcanic eruptions that lifted material to the surface from great depths. These materials were under great pressure and exploded upward, sometimes propelled to speeds of hundreds of kilometers per hour by expanding water and carbon dioxide gas. These strange eruptions formed long, thin “pipes,” conduits only a few hundred meters across but over 100 kilometers long. The volcanic eruptions must have shot some of the materials high into the air, but unlike other volcanoes, they did not deposit a mountain or other major amounts of material on the surface.

Instead, the uprushing material ripped debris off the vertical walls of the pipes, and the shafts of the pipes were left filled with unusual rock types that are classified as either kimberlite or lamproite depending on their chemical compositions; both types of rock sometimes include rare, high-pressure minerals, including diamonds. Just to make things interesting for prospectors, miners, and investors, some pipes contain a fabulous wealth of diamonds, and others contain none. Even the very richest deposits contain only about 1 carat (0.2 grams) of diamonds in a few tons of kimberlite, and most have much lower concentrations.

These pipes are often very hard to find because their surface features can be quite subtle. In the early 1990s, fabulously diamond-rich kimberlite pipes were discovered in shallow ponds covered with ice in northern Canada. The ponds filled hollowed-out depressions that had formed because the rocks in the pipes were softer and eroded a little faster than the surrounding rock.

Superdeep diamonds contain direct knowledge about the interior of our planet that otherwise would forever lie beyond our direct reach.

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Without the incredible volcanic diamond pipe process that elevates debris from great depths within the Earth, we would not know of diamonds except perhaps for the few that exist inside meteorites. We would have no diamond rings or diamond saws, and we would never say that anything “sparkles like a diamond.”

Because diamonds are so rare and valuable, diamond-bearing locations are nearly always off-limits to weekend prospectors who might wish to find a small diamond. There is one place, however, where the average person can just walk in, dig around for a day, and have at least a fighting chance of finding a diamond: the famous Crater of Diamonds State Park near Murfreesboro, Arkansas, where visitors find around a thousand diamonds each year. Although most of the diamonds are small, some are of exquisite quality. They are found in the weathered surface exposure of an ancient volcanic pipe. For a $13 entry fee, you can search the plowed surface of the pipe by screening buckets of soil, or, if you are really lucky, you might just spot a glistening diamond as you walk around.

In 2008, a tiny asteroid fell to Earth, breaking apart as it entered the atmosphere. Its fragments were fairly easy to find because they landed in a sandy desert in Sudan. Like some other meteorites, the fist-sized rocks contained small diamonds, which were formed by the shock pressures of asteroids colliding in space at high velocity. Many meteorites have features caused by high-speed collisions, but most do not contain diamonds made this way.

There may be other sources of diamonds in our Solar System. The four gas giants—Jupiter, Saturn, Uranus, and Neptune—have the right conditions in their atmospheres for “diamond rain”: plenty of carbon, and the pressure and temperature at some level in their extended atmospheres for gemstone-quality diamonds. (See “On Neptune, It’s Raining Diamonds,” September–October 2018.) And if diamonds are found in our Solar System, they must also be in other planetary systems with the right conditions for diamond formation.

One truly remarkable locale for the formation of diamonds is the dense interior of white dwarf stars, the end evolutionary state of most stars. Twenty years ago, astronomers at the Harvard–Smithsonian Center for Astrophysics found that the interior of the carbon-rich white dwarf BPM 37093 (nicknamed Lucy, after the Beatles song “Lucy in the Sky with Diamonds”) is crystalline diamond! Pulsations of the star’s brightness, caused by vibrations similar to the ringing of a bell, helped the researchers deduce its composition. The emerging field of asteroseismology can use these observational data to infer interior properties, and it appears that white dwarfs are an important repository of diamonds in the universe.

Growing Diamonds

Because of their value and utility, there has quite naturally been great incentive to create synthetic diamonds by industrial processes. At first, only tiny diamonds could be made because of the difficulty of creating the right conditions (after all, in nature the conditions necessary to create diamonds usually occur only deep beneath Earth’s surface or in the craters made by large bodies impacting from space), but over time new techniques have evolved to produce larger stones, and it is now possible to grow flawless diamonds of very large size and with an amazing range of colors. Strongly colored diamonds are exceedingly rare in nature, but that’s not the case for human-made diamonds. Designer diamonds of different colors can readily be made by including tiny amounts of other atoms in their structure. And the gems produced are of very high quality. The difficulty of distinguishing natural from manufactured diamonds has caused some consternation in the diamond industry, prompting companies such as Canadamark to inscribe their diamonds with a tiny, laser-engraved serial number so that they can be verified as true, natural Canadian gems.

Many synthetic diamonds are made at high pressure and high temperature in special presses, but there are other ways to make diamonds. Very small diamonds used as polishing and grinding powders can be commercially made in explosions, and diamond coatings and thin films are made by chemical vapor deposition (CVD) processes. Instead of high pressure, CVD uses a near vacuum in a microwave oven, where diamonds form directly from ionized gas. The conditions to make synthetic diamonds are also used to explore the nature of materials at high pressure deep inside Earth and other planets. Diamond anvils are hand-sized devices that squeeze two diamonds together to produce pressures similar to that at Earth’s center.

When we hear the word “diamond,” many of us think of gems, Audrey Hepburn standing in front of Tiffany’s, or Marilyn Monroe singing about her best friend. The gem business is the most famous part of the global diamond industry, but it represents only a part of the many ways that diamonds influence our lives. Go into almost any hardware store, and you will find abundant and relatively cheap tools that use less-than-gem-quality industrial diamonds. Diamond-based tools are becoming more and more affordable, and they are replacing others because of the superlative properties of diamond. An ordinary circular saw with an inexpensive steel blade that contains diamonds can cut granite, concrete, and ceramic tile almost like butter. The annual production of human-made industrial diamonds is billions of carats a year, and many sell for just a few dollars a carat.

Much of the utility of industrial diamonds centers on the stone’s hardness. Diamonds mark the top of the Mohs hardness scale, in which hardness levels are ranked in the pecking order of who can scratch whom. Diamond is a 10, and it can scratch sapphire, next in line at 9. Quartz is a 7 and can scratch ordinary glass, which is 5.5. Gypsum is a 2 and can be scratched by a fingernail.

Diamonds are also used in electronics and optics, and to form and polish eyeglass lenses and sometimes even large telescope mirrors. Eye surgery is done with tiny scalpel blades made of diamond, because diamond knives can be sharpened to nearly atomic sharpness, and diamond knives stay sharp longer than any other kind because of their hardness. Surgical diamond knives are made from high-quality gem diamonds and are quite expensive. You can, however, go to any hardware store and get a diamond-studded file to sharpen all your steel or even ceramic knives to a supersharp edge. Some people think that the electronics of the future may be based on thin diamond films, because diamond has properties that cannot be matched by the silicon-based electronic components in use at the present time.

Diamonds have superlative properties that are unmatched by any other known material. In addition, they provide a unique transport process that magically brings the deepest samples we have of our planet to the surface. Even more so than gold, diamonds have long held a special enchantment for humans. They are among the rarest things on Earth, and we wear them on our fingers, mount them in royal crowns, write songs about them, protect them in safes, and ogle them at Tiffany’s. Their ornamental, industrial, and geological properties enhance our lives in countless ways.

This article was excerpted and adapted from the authors' book The Sixth Element: How Carbon Shapes Our World, © 2024 by Princeton University Press.