The mystery behind the $ 4.3 million cosmic diamond

Geologists still cannot determine for sure whether the giant diamond called Enigma crystallized in the Earth's mantle or fell from space.

With a final auction amounting to $4.28 million, an unnamed buyer has become the new owner of the 555.55-carat Enigma black diamond. The auction results for this strange stone continue to ignite long-standing controversy about its origin, especially the hypothesis that Enigma may have come from outer space. Enigma and every other carbonado diamond formed in a mysterious event 2.6 - 3.8 billion years ago.

The Enigma Black Diamond.

Black, opaque, and filled with visible holes, the carbonado has a unique combination of chemical and physical features unlike any known diamond. They have only been discovered in two regions of the world, Brazil and the Central African Republic, and can reach extremely large sizes, including the 3,167-carat Sergio carbonado in Sergio, the largest diamond ever found. Enigma itself is not a small tennis ball-sized stone, says Thomas Stachel, a mineralogist specializing in diamond studies at the University of Alberta.

However, until recently, carbonados were recognized not for their beauty but for their hardness. Unlike the single crystals in traditional diamonds, carbonados are made up of an interlaced lattice of crystals, which increases their resistance to stress cracking and makes them useful as abrasives. Carbonado is used as a drill bit, can penetrate hard rock and is used with a grinding wheel to sharpen tools.

Carbonado's strange characteristics lead to many theories about their origin. The story of this super-hard mineral begins in two locations at either end of the Atlantic Ocean. Miners in eastern Brazil discovered black diamonds in the 1840s and named them carbonado after the Portuguese word meaning "burnt down" or "carbonized". Decades later, carbonado also appeared in the Central African Republic.

The carbonados at the two sites are so similar that there must be a connection between them, says Peter Heaney, a mineralogist at the University of Pennsylvania. Carbonado most likely formed when two landmasses joined together for more than a billion years and separated after Earth's last supercontinent, Pangea, broke apart 180 million years ago. Over the course of millennia, wind and water erased most clues to the origins of black diamonds, eroding the bedrock that contained the stones and scattering diamonds along the banks of ancient rivers.

Because there is no bedrock layer, scientists can only look for clues to explain the strange characteristics of black diamonds, but each stone seems to be associated with a different story. No one knows exactly where on Earth Enigma came from. The diamond is estimated to weigh more than 800 carats, or 160g, when it was acquired in the 1990s, according to the seller's representative. The stone is then cut into 55 faces, a process that takes up to 3 years due to its exceptional hardness.

Because diamonds crystallize under enormous pressure deep within the earth, they sometimes contain minerals from the planet's mantle such as garnets or blue olivine. But these minerals are not present in carbonado. Instead, geologists found a range of metals, such as the titanium nitride mineral called osbornite, which is most common in meteorites.

It is possible that carbonados form on carbon-rich stars or planets. Their fragments followed meteorites to Earth 3.8 - 4 billion years ago when the planet frequently collided with celestial bodies. Stephen Haggerty, a geophysicist who studies bone needles at Florida International University, first proposed this hypothesis at the 1996 conference of the American Geophysical Union. According to him, this is the only reasonable explanation of the carbonado's characteristics.

Other scientists aren't really sure. When Heaney began studying carbonado in the 1990s, the meteorite hypothesis seemed to be the logical explanation. But the more black diamonds he analyzed, the more Heaney assumed they formed in the Earth's mantle. But the proof that really convinced him was the enormous size of the many carbonado pellets.

Diamonds are found in meteorites or form due to the extreme heat and pressure in meteorite impacts, but they are all very small. Researchers still don't know the exact conditions for carbonado to form in the mantle. One confusing feature is that all carbonado diamonds have sponge-like holes. Most diamonds on Earth form at least hundreds of kilometers below ground under extreme heat and pressure. Small holes cannot appear in that condition.

Other studies indicate that the pores could be the result of carbonados crystallizing from extremely hot underground fluids, or that they contain minerals that were subsequently washed away. Heaney suggested that the holes may have once contained phosphate minerals. This mineral can destroy the diamond's reticular crystal structure and cause the diamond to darken when it decays. However, the interwoven network of holes makes it difficult for scientists to study any remaining matter because it is impossible to conclude that the mineral existed from the beginning or formed later.

While diamond is composed entirely of carbon, the element can come in either a heavy or a light isotope form. The isotope in carbonado is different from most ordinary diamonds, much lighter than diamonds that form deep in the Earth's interior and more like the organic carbon that constitutes life. Some scientists speculate that the light carbon isotope means that carbonados form from organic matter being pulled deep below the ground in subduction zones. 3 billion years ago when the carbonado formed, life was just beginning to form.

However, the clear answer will only come when we find more black diamonds. Possibly, a stone will contain material or be in bedrock that helps reveal its origin. Meanwhile, Enigma remains an outstanding testimony to the mysteries of the universe.