World's biggest rough diamond 'too big to sell'

The 1109-carat rough Lesedi La Rona diamond. Photo / Getty Images

This spectacular diamond may just be too big and impressive for its own good.

The three-billion-year-old rock - the largest discovered in more than a century - has failed to find a buyer at auction, leading to claims it is "too big" to sell.

It means the 1109-carat rock, which is the size of a tennis ball, may have to be cut into smaller pieces - an outcome its current owner is at pains to avoid.

"It's only the second stone recovered in the history of humanity over 1,000 carats. Why would you want to polish it?" Lucara Diamond Corp chief executive William Lamb told Reuters.

"The stone in the rough form contains untold potential ... As soon as you polish it into one solution, everything else is gone."

Auctioneer Sotheby's praised the stone's "exceptional transparency" ahead of its auction, predicting that it would be snapped up by an ultra-rich collector as a trophy piece.

It was expected to go under the hammer for more than $95 million ($US70 million) in June last year, but no buyer was found for the stone named Lesedi La Rona ("Our Light") in the national language of Botswana where it was mined.

Bidding stalled at $US61 million, well short of the $US70 million reserve, Reuters reports, despite the fact it is the second-highest gem quality rough diamond ever discovered.

David Bennett, chairman of Sotheby's jewellery division, called the discovery "the find of a lifetime" and the auction "unprecedented".

"Not only is the rough superlative in size and quality, but no rough even remotely of this scale has ever been offered before at public auction," he said at the time.

The largest diamond ever found was the 3106-carat Cullinan diamond, unearthed in South Africa in 1905. It was cut into nine pieces that form part of the UK Crown Jewels.

In 2015, the 12.03-carat polished "Blue Moon" diamond sold for a record-breaking $US48.5 million ($AUD64.75 million) in Geneva.

Controversial Hong Kong billionaire Joseph Lau picked it up as a gift for his seven-year-old daughter, prompting the nickname "the Suri Cruise of Asia".