Petra Diamonds (LON:PDL) has sold the "exceptional" 122.52 carat blue diamond found at the famous Cullinan mine in South Africa in June this year for US$27.6 million, or US$225,269 per carat.
The buyer is a beneficiation partnership consisting of Petra, with a 15% stake, and an undisclosed polishing partner, with 85%.
The partner will pay US$23.5 million immediately - a figure that Petra will record as revenue in its first quarter 2015 numbers.
Petra chief executive Johan Dippenaar said: "We are pleased to have concluded this arrangement which records an excellent sales value for the rough stone, and also gives Petra exposure to the uplift in value post beneficiation.
"We look forward to providing the market with further information on the stone's journey at the appropriate time in the future."
Investec said: "The value uplift from the cutting and polishing can be very material and the sales structure agreed is an interesting one.
"Petra therefore realises a meaningful cash injection now and exposure to upside from polishing whilst mitigating the risk of damage to the stone from the cutting and polishing process. The value achieved is ahead of what our analyst modelled of US$20m but some had been speculating over the potential for US$30-40m and higher."
The Cullinan mine has produced just under 800 stones weighing more than 100 carats, over 130 stones weighing more than 200 carats, and around a quarter of all diamonds weighing more than 400 carats.