Petra Diamonds has sold two special stones from its Cullinan mine in South Africa into a partnership with diamond sourcing and supply company Stargems.

The South African subsidiary of Dubai-based Stargems Group purchased a 342.92-carat, white, type IIa diamond for an upfront payment of $10 million, Petra said Wednesday. The same buyer also paid $3.5 million for an 18.30-carat, blue, type IIb diamond.

Stargems will cut and polish both stones in South Africa before putting them up for tender. Petra will receive 50% of any profit from the polished-diamond proceeds minus the manufacturing costs, it explained.

“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,” said Petra CEO Richard Duffy. “We are delighted that both stones will be manufactured in South Africa, and it is fitting that we will be working with Stargems, who specializes in the sourcing and supply of the finest diamonds to customers across the world.”