Lesedi la Rona diamond estimated at $70m

At your common or garden auction, you can rely on anything from 50 to 500 lots going under the hammer. If you miss one Meissen porcelain figurine, another one will be along in a second. But at Sotheby’s in London on June 29, blink and you will miss it: there is only one thing for sale.

The sole lot is a 1,109-carat rough diamond, Lesedi la Rona (pictured above), which means “our light” in the Tswana language of Botswana, where it was found. It was the largest diamond dug up for more than a century, roughly the size of a tennis ball, according to Sotheby’s.

Sotheby’s estimates its value at $70m. But if it matches the world record for a rough diamond per carat — $77,649 — it would fetch over $86m.

David Bennett, chairman of Sotheby’s jewellery division, says he envisions three types of people who might buy the stone: the professional who will cut and sell it; the private individual who will have it cut to their own taste; or someone who will keep the stone as it is, “one of the rarest objects in the world”.

“I was trembling,” Mr Bennett says, when he first laid hands on the diamond, and he talks about it in reverent, appreciative tones. He is speaking from Antwerp, where the stone has stopped on a global tour put on by Lucara, owners of the mine it came from.

Mr Bennett points out that the stone’s emergence was due to “randomness”: a volcano happened to belch it up to 80m below ground, when it was formed 3bn years ago 100 miles beneath the Earth’s surface.