The world's largest black diamond

Enigma is a black diamond (carbonado) and is the largest cut diamond in the world, according to Guinness World Records, weighing 555.55 carats (111 grams) with 55 facets.

In addition to its unusual color and unique shape, one of Enigma's most special features is that it may have originated from space. According to auction house Sotheby's announcement, Enigma was created by a meteorite impact or came from an asteroid colliding with Earth. However, black diamond experts are not sure about this.

Black diamonds have high concentrations of opaque minerals, especially graphite, a dark gray form of carbon with a hexagonal structure and metal sulfides, according to Peter Heaney, a professor of geosciences at the University of Pennsylvania. Sotheby's describes the Enigma as a carbonado diamond. However, not all black diamonds are carbonado. Instead, carbonado is a type of black diamond that is both polycrystalline and hollow. Carbonado diamonds also contain uranium-rich phosphate, which creates a halo of radiation around holes in the surface. That characteristic makes them highly absorbent of white light, completely opaque and darker than many other types of black diamonds.

Carbonado is super hard and super durable due to its polycrystalline and porous properties. This makes them perfect for industrial use, for example drill bits used in the oil and gas industry to penetrate hard volcanic rock. Judging by the size, it is likely that the Enigma is indeed a carbonado, but it is difficult to say for sure because the diamond has been cut.

All known carbonado diamonds have been found in Brazil or the Central African Republic, dating back 2.6 - 3.8 billion years. During this time period, the two countries belonged to the same supercontinent named Rodinia. Scientists have discovered that black diamonds were born from meteorite impacts in the past, but they are usually very small. "I would be very surprised if a diamond as large as the Enigma originated from a meteorite impact," Heaney shared.

Some other theories speculate that carbonado diamonds may exist in some asteroids that crash into Earth or form from powerful stellar explosions called supernovae, but there is not enough evidence to prove it. Instead, Heaney suggested that the carbonado's origin was the Earth. However, the exact mechanism that creates black diamonds has not been determined. Most diamonds are born when high pressure in the Earth's mantle compresses organic carbon. But the oldest carbonado diamonds may have existed before life on Earth, so researchers aren't sure they formed this way.