Discovered at the Cullinan Diamond Mine in South Africa, the diamonds have been sold into a partnership with Stargems, a subsidiary of the international diamond company Stargems Group.
Through the transaction, Petra is set to receive an upfront payment of $10m (£7.28) for the 342.92 carat diamond and $3.5m (£2.55m) for the 18.30 carat stone.
Moreover, the group will retain a 50% interest in the profit uplift of the polished proceeds of both the diamonds after costs.
Richard Duffy, CEO at Petra, said: “These two diamonds are wonderful examples of the very high quality and rare white and blue diamonds that are so well known from the Cullinan Diamond Mine.
“We are delighted that both stones will be manufactured in South Africa and it is fitting that we will be working with Stargems, who specialise in the sourcing and supply of the finest diamonds to customers across the world.”
Shailesh Javeri, chairman Stargems Group, added: “It is with great honour that we have entered into a partnership arrangement with Petra on these two diamonds from the world-renowned Cullinan Diamond Mine.
“The stones will be beneficiated in South Africa at our Stargems cutting factory in Johannesburg and we look forward to working closely with Petra during the manufacturing process to reveal the eventual polished gems.”