Petra Diamonds recovers 138.57 carat white diamond from Cullinan

The company said the diamond will be offered for sale as part of its next sales process late this month in Johannesburg.

RBC Capital Markets said the announcement was a small positive for the shares.

“Depending on yield potential, we expect the stone to realise $4m-$7m, which may lead to a slight increase in consensus full-year 2017 earnings per share forecasts.

“We also question whether this stone and the large white sold in the June tender are evidence of the early contribution of undiluted ore in the process mix; if so the incidence of larger stones should increase.”

Investec said this was "a nice result following the recent 121.6 carat white stone recovered in June that sold for $6m".

"Perhaps further indication of increased fresh ore being processed delivering better grades and more regular special stones. If a similar value per carat is achieved for this latest stone the value could be around $7m, or about 1p/share."

At 1023 BST, Petra shares were up 1.7% to 119.75p.