The volume processed from the KV2 kimberlite pipe fell 12% from the previous quarter to 34,247 tonnes, which the company said was a good results considering the downtime due to a crusher fault and bad weather.

The average mined grade fell to 2.45 carats per hundred tonnes from the previous quarter's 3.8cpht, although BlueRock said grades were expected to improve as production went deeper.

It did not give the volume of diamonds recovered, but said 875.31 carats were sold during the three-month period, down 44% from the March quarter, at an average revenue of US$305.34 per carat.

With production for the first half of the year totalling 73,028t, up 81% year-on-year, BlueRock put guidance for the second half of the year at 150,000-175,000t.

The company started mining at the second kimberlite pipe, KV1, in June, which has an inferred grade of 6.3cpht and should add to production volumes.

BlueRock CEO Adam Waugh said the quarter was successful, despite the five week of downtime, with diamond quality continuing to be "exceptional".

"Additionally, to make up the lost tonnage from H1 due to unanticipated downtime and to provide crushed stockpile for the wet season in H1 2019, we are focused on increasing our crushing capacity from the existing circuit to fully unlock the value in our 8 million tonne inferred mineral resource," he said.

BlueRock's shares were trading at 1.17p (US0.89c) Friday, up 2.12%.