When a new employee first spotted the tennis-ball-sized Lesedi la Rona diamond in a Canadian mine, he didn’t think much of it.

It was November at the Lucara’s Karowe mine in Botswana, and the employee thought finding a rock that big was perfectly normal, CEO William Lamb told the Canadian Press.

“He’d only been working for us for two weeks, so he sees the thousand carats and goes ‘oh, this must be normal.’ He didn’t tell anybody. It was only when the chief saw it that there was this huge commotion on site.”

The world is now making a commotion. The tennis-ball-sized diamond the employee thought was so average is anything but.

The gem is three billion years old and 1,109 carats, which makes it the second-largest gem-quality rough diamond ever found. Such a find comes around only once a century, apparently, because the last time anyone found a diamond that massive and valuable was in 1905.

That record-breaking gem was the 3,106-carat Cullinan, found in South Africa, cut into nine pieces and incorporated into the U.K. Crown Jewels. The mine worker who spotted it, shining in the side of the pit, dug it out with a pen knife.

The Lesedi la Rona, however, almost wasn’t found at all. Lamb said it made it through the modern mining process before it was spotted: drilling, blasting, excavation, crushing, and mechanized sorting.

“We put an estimate on it in excess of $70 million, so that is the sort of area we are expecting to go for, but frankly it is difficult to tell — what is the largest diamond in the world worth?”
Sotheby’s will offer the tennis-ball-sized diamond for sale on June 29. This will mark the first time they’ve auctioned a rough diamond this large, he told the Evening Standard.

“The Lesedi la Rona is simply outstanding and its discovery is the find of a lifetime,” he said. “Every aspect of this auction is unprecedented. Not only is (it) superlative in size and quality, but no rough even remotely of this scale has ever been offered before at public auction.”

It’s also been described as a gem of “exceptional transparency” by Sotheby’s. The eventual owner can either keep it rough, cut and polish it, or trim the tennis-ball-sized stone into smaller pieces. It could be the largest top-quality diamond ever cut and polished, but Lamb hopes whoever buys it will leave it just as it is.

The soon-to-be second-most expensive stone, the Blue Moon Diamond, was 12.03 carats and purchased by Hong Kong billionaire Joseph Lau for his 7-year-old daughter.