The second-largest gem-quality rough diamond ever recovered could sell for more than $70m (£50m) when auctioneer Sotheby's puts it up for sale in London on 29 June. The Lesedi la Rona diamond is the size of a tennis ball and is the largest discovery of its kind in more than a century.

The 1,109-carat gem was excavated in Botswana in November 2015 at a mine owned by Canada's Lucara Diamond Corporation. William Lamb, the company's chief executive called it "the most valuable item on the planet".

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David Bennett, the chairman of the auction house's jewellery division, said the sale of such a valuable item was unprecedented. "The Lesedi la Rona is simply outstanding and its discovery is the find of a lifetime," he announced. "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."

Lesedi la Rona means "our light" in Tswana, one of Botswana's official languages which is used by almost 90% of its population. The dimensions of the 3bn-year-old diamond stand at 65mm × 56mm × 40mm.

The 3,106-carat Cullinan diamond is the largest discovery of its kind to date after it was unearthed at the Premier Mine in South Africa in 1905. Nine principal stones were cut from the diamond and were set in brooches, a ring and a necklace while the remaining two form part of the Crown Jewels, according to the Royal Collection Trust.

Last year Hong Kong billionaire and property tycoon Joseph Lau paid a record $48.5m for the 12.03-carat "Blue Moon" diamond at an auction in Geneva as a gift for his seven-year-old daughter. The night before that purchase, Lau parted with $28.5m to acquire a 16.08-carat pink diamond from a Christie's auction. The gem was subsequently renamed "Sweet Josephine".

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