A billion-year-old black diamond thought to have come from outer space is going up for auction.

Named The Enigma, it is believed to be the largest cut diamond on earth and may have formed from a meteoric impact.

At 555.55 carats the gem is considered extremely heavy for a diamond, weighing about the same as a banana.

The rock is expected to fetch over £4.4 m ($6m) at a Sotheby’s auction in London by the time online bidding closes next week.

The gem is a carbonado, one of the toughest forms of natural diamond.

Black diamonds are usually about 2.6 to 3.2 billion years old – a time before dinosaurs existed.

The Earth itself is around 4.65 billion years old, so not much older than black diamonds.

Carbonados are extremely rare and have only ever been discovered in Brazil and the Central African Republic.

Because they contain osbornite, a mineral found only in meteors, they are believed to originate from space.

But their precise origins are shrouded in mystery.

Aaron Celestian, a geologist at the Natural History Museum of Los Angeles, said that carbonados “could have formed super deep within the Earth’s interior, far deeper than what we know already of diamonds.

“There are hypotheses that suggest they were formed at impact sites where a large asteroid fell on earth,” he told Reuters news agency.

A competing theory is that black diamonds may have formed in outer space before falling to the Earth.

Sotheby’s is describing the sale as a “once-in-a-lifetime opportunity to acquire one of the rarest, billion-year-old wonders known to humankind.”

As well as weighing exactly 555.55 carats, the diamond has been cut to display 55 facets or faces.