Giant diamond too big for collectors to swallow

In the world of diamond mining, it turns out that some stones are too big to sell.

Canadian company Lucara Diamond will have to cut its tennis-ball-sized rough diamond to find a buyer, industry insiders say.

Sotheby's was unable to attract a buyer when it auctioned the world's biggest uncut stone last year.

It's not the ending that Lucara CEO William Lamb wanted for his 1109-carat stone, named "Lesedi La Rona", or "Our Light" in the language of Botswana, where it was mined.

"It's only the second stone recovered in the history of humanity that's over 1000 carats. Why would you want to polish it?" said Lamb.

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

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.

Bidding for the 2.5 billion to 3 billion-year-old stone stalled at $61-million - it was reserved at $70-million.

"When is a diamond too big? I think we have found that it's when you go above 1000 carats," said a mining analyst.