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]

The reputation of diamonds mined in Africa has been tarnished in recent decades by rebels in strife-torn countries who forced people to mine them and then sold the so-called "blood diamonds" to raise money to buy arms.

But the Kimberley Process Certification System, a United Nations-backed program that was set up in 2002 following devastating civil wars in Angola, Sierra Leone and Liberia, has made trafficking in conflict diamonds much more difficult.

Sotheby's said independent reports by experts showed the Lesedi la Rona could have the potential to yield the largest, top-quality diamond ever seen once it has been cut and polished.

"It's worthwhile for people to come and look at it because you probably won't be seeing it again in two or three year's time," Bennett said. "It may very well be cut up into all these wonderful famous stones."