VANCOUVER (Reuters) - A tennis ball-sized rough diamond unearthed nearly two years ago by Canada's Lucara Diamond Corp has been sold for US$53 million (S$72 million), the Vancouver-based company said on Monday.

The 1,109-carat stone, which is the world's largest uncut diamond, was bought by high-profile British diamond dealer Graff Diamonds for US$47,777 per carat, and will now be cut and polished, Lucara said.

The price paid was an "improvement" on the highest bid received for the diamond at a Sotheby's auction in June 2016, Lucara chief executive and president William Lamb said.

He had gambled that ultra-rich collectors, who buy and sell precious art works for record-breaking sums at auction, would do the same with a diamond in the raw.

The unprecedented bet failed when bids did not meet the auction reserve price and the diamond was not sold.

The stone, named Lesedi La Rona or Our Light in the national language of Botswana where it was mined at Lucara's Karowe mine in November 2015, is said to be 2.5 to 3 billion years old.

The rough diamond is second in size only to the 3,106.75 carat Cullinan, which was cut into 105 diamonds, including several British Crown Jewels.