The rarest blue–green diamond on Earth is up for auction – and experts predict it could fetch up to £9.4 million ($12.8 million). 

Dubbed the 'Ocean Dream', the diamond was discovered in its rough form in Central Africa in 2002. 

The original crystal weighed a whopping 11.17 carats, before it was cut down to preserve and maximise its stunning colour.

The final gem now weighs 5.50 carats, making it the largest diamond of this colour ever recorded.

'The Ocean Dream is, quite simply, in a category of one,' said Tobias Kormind, Managing Director of 77 Diamonds.

'This is the largest fancy vivid blue–green diamond ever certified by the GIA in its entire history. 

'There is no other diamond of comparable colour and size anywhere on record.

'It is a perfect blend of two of the rarest diamond colours to exist.'

While most of us think of diamonds as colourless, they can actually come in a range of colours, based on the trace elements or radiation they're exposed to during formation.

The Ocean Dream is categorised as a 'fancy vivid blue–green diamond' by the Gemological Institute of America (GIA). 

This colour is a result of prolonged exposure to natural radiation deep within the Earth over millions of years, and is so uncommon that it borders on geological singularity. 

'I could spot it from across the room!' said Tom Moses, Executive Vice President and Chief Research and Laboratory Officer of the GIA.

'I have never seen a natural blue–green diamond of this intensity of color, size, and it is a Type Ia, which certainly makes it a unicorn.'

While the original rough form weighed 11.17 carats, the diamond was cut down into a triangular brilliant–cut, which shifts between blue and green depending on the light. 

In 2003 – one year after its discovery – the diamond was one of seven exhibited at the Smithsonian Museum in Washington DC.

D. Jeffrey Post, curator of the National Gem Collection at the Smithsonian Natural History Museum, said: 'The Ocean Dream was one of seven rare and spectacular diamonds in our internationally acclaimed "Splendor of Diamonds" exhibition, it was one of the most popular special exhibits ever hosted at our museum.'

Eleven years later, the diamond was sold at auction in Geneva for £6.6 million ($9 million). 

Fast–forward to today, and the diamond is back on the auction block at Christies, Geneva, where experts predict it could sell for even more. 

Mr Kormind, from 77 Diamonds, explained: 'When legendary fancy coloured diamonds come to auction, the market responds.

'The 14.62 carat Oppenheimer Blue sold for $57.5 million in 2016 and the 12.03 carat Blue Moon of Josephine for $48.4 million in 2015. 

'The Ocean Dream is a different proposition entirely – rarer in colour than either, and seen at auction only once before.

'For the right collector, this is a once–in–a–lifetime acquisition, and I believe its high estimate of CHF 10m (approx $12.8m) could prove conservative indeed.'

The news comes shortly after an 'astounding' half–pink diamond was unearthed in the Karowe Mine in Botswana, weighing a whopping 37.41 carats.

The diamond is one inch long and has a 'sharp' boundary between the dusky pink and colourless sections.

Jewels this colour are incredibly rare because temperature and pressure conditions have to be just right for them to form.

Experts believe it could be one of the most important pink diamonds in history.

Oded Mansori, co-founder of diamond-cutting firm HB Antwerp, said: 'This stone has the potential to become one of the most important pink diamonds ever polished.

'Its intensely rich coloration is a testament to the geological uniqueness of the Karowe Mine.

'And our deep understanding of the manufacturing process will allow us to capture its vibrant color to the fullest extent possible.'