“This stone is one of the most exceptional stones recovered at Cullinan during Petra’s operation of the mine. The stone is an outstanding vivid blue with extraordinary saturation, tone and clarity, and has the potential to yield a polished stone of great value and importance.” the company said.

In 2013 Petra had unearthed a 25.50 carat blue diamond which was sold for £10.3m ($16.9m). Another similar 26.6-carat blue rough diamond discovered by the company in May 2009 was cut into a near perfect stone and fetched just under $10m at a Sotheby’s auction.

The current world record for the value per carat has been set by another deep-blue diamond from Cullinan, auctioned for $10.8m in 2012. Owing to the price rises in the past 2 years, everyone is looking forward to what this will be cut to and the price it will fetch.

Cullinan mine has produced hundreds of large stones and is famed for its production of blue diamonds – among the rarest and most highly coveted of all diamonds. The mine was acquired in 2008 by Petra Diamonds, which also operates in Botswana and Tanzania.