Petra's revenue in the six months to December was US$264.7 million, up by 49% year on year. This increase was driven by high returns from selling exceptional stones, which delivered revenues of US$77.9 million, an increase of 93% year on year. Revenue was also boosted by higher diamond prices.

Production was up 2% to 1,777,424 carats. Petra maintained full-year 2022 gross production guidance of 3.3 to 3.6 million carats.

The company's balance sheet as of the end of December showed consolidated net debt of US$153.6 million, down by 24% from three months earlier.

"Looking to the numbers, we suspect the impressive net debt performance was aided given the better-than-expected revenues and working capital inflow from no diamond debtors at 31 December, something that contributed c.US$30 million of the US$48 million difference," Peel Hunt said in a note.

"More working capital benefits are likely to have further aided the cash release, but at this stage we also cannot rule out lower than expected cash costs."

"We believe this is a performance that will allow investors to look to the future for Petra," Peel Hunt said.

Cullinan is also making progress in mitigating operational problems at Cullinan in South Africa.

"As previously announced, the Cullinan mine experienced convergence in Tunnel 41 on the eastern side of the C-Cut block cave during the period, impacting 18 of a total of 187 draw points," Petra said. "Mitigating steps were rapidly introduced which are now largely offsetting the impact on Cullinan's production for FY 2022, resulting in the mine's production guidance remaining unchanged at 1.7 to 1.9 million carats for FY 2022."

Petra downplayed the effect of the Omicron COVID wave on operations. "There has only been a limited impact on our South African and Tanzanian operations during this wave, which is generally perceived to have now passed its peak in South Africa."

Petra shares were up 19% at 89.00p at 16.35 London time.