1,109-carat diamond to be auctioned in London

A model displays the 1109 carat "Lesedi La Rona" diamond at Sotheby's in the Manhattan borough of New York, US, May 4, 2016. [Photo/Agencies]

"It really just soared off the scale of rare into something just, one off, it's just unique," he said.

Unearthed in Botswana in November 2015 by Canadian mining company Lucara Diamond Corp., the gigantic gem is about the size of a tennis ball and is believed to be between 2.5 billion to more than three billion years old.

The Lesedi La Rona's color and transparency are typical of a rare and coveted subgroup called Type IIa diamonds, according to a study by the Gemological Institute of America.

Bennett said it was second only in size to the Cullinan Diamond, which was discovered in 1905 in South Africa and weighed more than 3,000 carats. The Cullinan Diamond was later cut into several smaller stones.