A 20.08 carat blue diamond was found in Cullinan on Monday.

Petra Diamonds reported that the diamond is rated as a Type IIb stone, which are among the rarest and most valuable diamonds in the world.

It is only slightly lighter than the biggest blue diamond ever found in diamond-rich Botswana.

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On Monday, a rare 20.08 carat blue diamond was found in the Cullinan Diamond Mine, 40km east of Pretoria, Petra Diamonds announced.

The gem quality diamond is classified as a Type IIb stone, which is among the rarest and most valuable diamonds. The diamonds, which have very low levels of impurities, make up about 0.1% of all natural diamonds.

The diamond is only slightly lighter than the biggest blue diamond (of 20.46 carat) ever found in diamond-rich Botswana. Named "Okavango Blue", that diamond was found in the Orapa mine, the world's largest diamond mine by area.

Bloomberg reported that a 30-carat blue diamond from Cullinan was sold for $26m (around R400m at current exchange rates) in 2014.

Earlier this year, two massive yellow diamonds of more than 70 carats were found in Lesotho.

Petra Diamonds, which is listed on the London Stock Exchange, is South Africa's second largest diamond operation by production and has two other mines, in the Northern Cape and the Free State.

The company is currently battling a massive debt burden, and its share price hit a record low earlier this month after it wrote down the value of its mines.

“Today's recovery not only demonstrates the quality of Petra's asset base, as the Cullinan diamond mine remains a significant source of rare blue diamonds, but it again confirms the prevalence of exceptional stones in the Cullinan orebody as well as the ability of the mine's plant to recover the full spectrum of diamonds," it said in a statement.

The company will report production and sales results for the quarter ending 30 September 2019 on October 21st.

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