Diamonds in the Sky

Glittering stars in the night sky aside, scientists have long known that some diamonds hail from the heavens. In 1981, for example, when Smithsonian researchers tried to cut through a large iron meteorite that had crash-landed in the Allen Hills of Antarctica, the sawteeth on their blade got all chewed up. Subsequent X-rays showed that the stone was riddled with microscopic diamonds, the hardest substance known. The scientists theorized that the meteorite's diamonds were born during a cataclysmic collision out in the asteroid belt.

Cosmic gems

Other meteoritic diamonds apparently hail from deep space. In 1987, a team of researchers headed by Edward Anders and Roy Lewis of the University of Chicago reported the discovery of meteorite-embedded diamonds so miniscule that trillions could fit on the head of a pin. Unlike the Smithsonian diamonds, these microscopic crystals contain an isotopic mixture of xenon gas not found on Earth. "It seems necessary to invoke an extra-solar origin for the diamond," the scientists concluded in a paper published in Nature, indicating a birth outside our solar system.

Indeed, the team proposed that the lucent crystals formed in the atmosphere of a "red giant" or dying star before it collapsed and exploded billions of years ago. The supernova would have sent the diamond-studded material far out into space, where in the fullness of time some pieces eventually fell to Earth. If this scenario is correct, the researchers said, then interstellar dust may be peppered with tiny diamonds.

Still other diamonds are apparently created during the fiery instant when meteors and meteorites slam into Earth. In the 1960s, scientists discovered more microscopic diamonds in the remains of the vast Canyon Diablo meteorite, which formed Meteor Crater in Arizona. The diamonds are sand-grain-sized, only hundredths of an inch across. Other crater-related diamonds are larger. In the 35-mile-wide Popigari crater in Siberia, the result of a huge impact 35 million years ago, Russian researchers unearthed polycrystalline diamond clusters reaching nearly half an inch across.

Many of these impact-spawned diamonds bear the cubic structure of ordinary, Earth-grown diamonds. But analysts studying the Canyon Diablo diamonds found that up to a third of them bore a hexagonal atomic structure never before seen in diamond. Mineralogists named the new hexagonal variant of diamond lonsdaleite after the British mineralogist Dame Kathleen Lonsdale, who helped advance the study of natural diamond crystals.

Origins unclear

Today, more than 30 years after the discovery of the Canyon Diablo diamonds, scientists still debate how such mini-diamonds form. Some suspect they were wrought in the vacuum of space by vapor deposition, a process that specialists can use to make synthetic diamond here on Earth. Others maintain that, within the hurtling meteorite itself, carbon atoms (or, in a minority opinion, grains of meteoritic black graphite) transformed instantly into diamond during the extraordinary heat and shock of impact.

Even nearby planets may be churning out diamonds.

Whatever the origin of meteorite diamonds, some scientists believe they have found evidence that the colossal cloud of dust thought to be thrown up into the atmosphere in the wake of such impacts may spread newly formed diamond dust all around the world. In 1991, Canadian geologists David B. Carlisle and Dennis R. Braman reported finding Lilliputian diamonds embedded in a layer of sediment 65 million years old—right at the time when many scientists believe a giant meteor slammed into Earth and precipitated the extinction of the dinosaurs.

Can these miniature diamonds, which are so fine-grained that the researchers deem them the result of a collision, serve as an indicator of this ancient catastrophe, much as the famous iridium layer has done? Scientists won't be able to say without further study, but the idea holds promise. (In the 1998 book The Nature of Diamonds, the geologist George Harlow and two Russian colleagues wrote simply, "This subject is very new, and many exciting discoveries have yet to be announced.")

black diamonds

Outer space may also be the birthplace of the mysterious black diamonds known as carbonados. From the Portuguese word for burned or carbonized, carbonados were first found in Brazil in the 1800s and have since turned up elsewhere, most notably in central Africa. Unlike the clear diamonds of engagement rings, which are single crystals, black diamond consists of aggregations of individual crystals, which lend the gem its dark color. The largest diamond ever found was a carbonado from Brazil. Named Sergio, the stone weighed 3,167 carats. (One carat equals one-fifth of a gram.)

The origins of carbonados have long baffled scientists. Black diamonds don't adhere to the rules of diamond mineralogy, and they don't occur in the usual places where clear diamonds turn up. Even so, scientists initially believed they must have been fashioned in the same conditions under which clear diamonds are thought to form. That is, they were crafted deep within the Earth, 100 to 300 miles down, when intense heat and pressure transformed carbon into diamonds, which volcanic eruptions then lofted to the surface.

But that theory suffered a blow when scientists examined the carbon isotopes of black diamonds. (Isotopes are species of a chemical element that reside in the same place on the periodic table but have different atomic weights and physical properties.) Unlike clear diamonds, black diamonds feature ratios of the two most common carbon isotopes in the Earth's crust, carbon-12 and carbon-13. These isotopes characterize surface carbons rather than those found in the Earth's depths.

Wherefrom?

This finding helped lead to a new theory of carbonado formation. In 1985, Joseph Smith of the University of Chicago and J. Barry Dawson of the University of Sheffield in England suggested in an article published in the journal Geology that large meteor impacts in the Precambrian Era (570 million years ago back to Earth's beginning some 4.5 billion years ago) formed the black diamonds we find today. Scientists had long deemed carbonados quite old, because the streams where they are typically uncovered cut through geologic strata dated from one to more than two billion years old. In fact, recent atomic measurements of black diamonds have placed their origins at nearly four billion years ago, a time when a constant barrage of giant meteors battered the Earth.

In the 1990s, other scientists showed that Brazilian and African carbonados bear similar isotopes of carbon and nitrogen, suggesting a common origin, while still others provided theoretical and physical evidence that black diamonds could have arisen during the extreme shock and heat of a meteor impact. But why, some scientists wondered, had no unambiguous evidence ever been shown for craters associated with carbonados?

As for tapping the riches of any diamonds from space, don't hold your breath.

Geologist Stephen Haggerty of the University of Massachusetts at Amherst had an idea why, and he shared it with a dumbfounded audience at a 1996 American Geophysical Union meeting in Baltimore. Carbonados were born not on Earth, either the way regular diamonds are or by meteor impact, he said. Rather, they originated in dying stars, when shock waves from exploding red giants crushed carbon into dense aggregations of black diamond and sent them hurtling into deep space. Eons later, the sun's gravity lured some of this material into our solar system, where blocks of it slammed into our atmosphere, shattering into the fragments we find strewn over select areas today, perhaps billions of years after they formed.

Lucid in the sky with diamonds

Even nearby planets may be churning out diamonds. In fact, planetary scientists say that Uranus and Neptune, the seventh and eighth planets from the sun, may rain diamonds, which then pile up miles-thick at the planets' cores.

Uranus and Neptune are each nearly four times the size of Earth. Scientists believe that, beneath an outer layer of hydrogen and helium, the gaseous atmospheres of both planets contain 10 to 15 percent methane, a hydrocarbon. Deep within the extremely dense atmospheres, above a rocky core, these planets are also thought to bear temperatures ranging from 3,000 to 12,000°F and pressures varying from 200,000 to 6,000,000 times the pressure of our own atmosphere.

In other words, possibly ideal conditions for producing diamonds.

With this in mind, a team at Lawrence Livermore National Laboratory tested in the early 1980s what would happen to methane under intense pressure. One of the team's leaders, Marvin Ross, had calculated that the gas would separate into hydrogen and carbon at temperatures above 3,000°F and pressures exceeding 200,000 Earth atmospheres. The carbon atoms would be squeezed so tightly together that they would become diamonds, he theorized.

To find out if he was right, Ross and his team used a gas cannon to severely compress and shock methane samples. Resulting data, they later reported, indicated the fleeting formation of diamonds in the instant before the target material evaporated, and recent experiments at several labs support the predictions.

Reaping the benefits

As for tapping the riches of any diamonds from space, don't hold your breath. Earth-hitting meteorites that either bear or engender diamonds are few and far between, and unlike diamonds you're likely used to, their associated diamonds often cannot be seen, much less admired, with the naked eye. Black diamonds, for their part, are rare and are primarily used for industrial purposes. And the challenges of harvesting any diamonds on Uranus and Neptune, which are roughly 1,700 and 2,720 million miles away from Earth, respectively, are as clear as the Koh-i-Noor diamond. Alas, scientific understanding is the primary beneficiary of diamonds from the heavens.