“The recoveries demonstrate the prevalence of these types of stones in the Cullinan orebody as well as the ability of the mine's plant to recover the full spectrum of diamonds,” Petra said.
The company, which appointed last month former gold miner Richard Duffy as chief executive, said that both stones would be included in its upcoming March tender.
Shares climbed as much as 10.5% after the announcement, closing at 21.2p in London on Friday.
Petra has been seeking to turn around its fortunes after piling up debt to expand Cullinan, where the world’s biggest-ever diamond was found in 1905.