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.

They are unusual for being the color of charcoal and full of frothy bubbles. The diamonds, which can weigh in at more than 3,600 carats, can also have a face that looks like melted glass. The largest diamond ever found was a carbonado from Brazil; named Sergio, the stone weighed 3,167 carats.

A rough black diamond

Black diamonds don't adhere to the rules of diamond mineralogy, and they don't occur in the usual places where clear diamonds are found. 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—that characterize surface carbons rather than those found in the Earth's depths.

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 scientific journal Geology that large meteor impacts in the Precambrian Era (roughly 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 found 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?

Now a team led by geologist Stephen Haggerty of Florida International University in Miami has presented a new study suggesting that the odd stones were brought to Earth by an asteroid billions of years ago.

The scientists exposed polished pieces of carbonado to extremely intense infrared light. The test revealed the presence of many hydrogen-carbon bonds, indicating that the diamonds probably formed in a hydrogen-rich environment—such as that found in space.

Stephen Haggerty had an idea why no crater associated with carbonados was found. The 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.

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