London-listed Petra Diamonds increased its production by 21% to 2.67-million carats in the 2013 financial year and said on Tuesday that output was expected to climb to between 2.9-million carats and 3-million carats in 2014.

The Northern Cape-based Finsch mine has gone some way towards justifying its R1.4-billion price tag, delivering 1.42-million carats in the year ended June 30 – more than half of the total output.

However, grades dropped to 34.1 ct per hundred tonnes (cpht) for run-of-mine (RoM) ore and 20.1 cpht for tailings at the former De Beers operation, which Petra said was the result of a lower plant cutoff grade, in line with its strategy to benefit from the increased recovery of the higher-quality, smaller diamonds at Finsch. "This also served to increase gross revenues and contained value per tonne,” commented CEO Johan Dippenaar.

Gross mine revenue increased 27% to $402.7-million for 2013, primarily owing to increased volumes, coupled with the sale of a 25.5 ct blue diamond from Cullinan, in Gauteng, for $16.9-million, and a 35.8 ct stone from Williamson, in Tanzania, for $1.3-million.

Petra sold 13 stones exceeding $1-million each during the year, for total revenue of $39.5-million. Carats sold increased by 22% to 2.54-million carats, in line with the increasing trend of production.

The successful start-up of the rebuilt treatment plant at Petra’s Williamson operation also boosted the company’s full-year production figures, with the Tanzania-based kimberlite more than tripling output from 42 855 ct in 2012 to 150 342 ct in 2013.

In contrast, the iconic Cullinan mine produced less in the financial year as a strike in October, and RoM grade, delayed the roll-out of the tailings expansion project. Cullinan only treated 1.5-million tonnes, rather than the planned 2.7-million tonnes, consequently curtailing production to 868 975 ct for the year.

Diamond production at the Koffiefontein mine, in the Northern Cape, declined by 13% to 34 800 ct, while underground production continued to be supplemented with production from the Ebenhaezer satellite pipe, where waste stripping activities were progressed in the fourth quarter to establish access to higher-grade ore. "As a result, Koffiefontein is now in a position to deliver improved results for the 2014 financial year,” Dippenaar said.

At the Kimberley Underground operation, also in the Northern Cape, the ramp-up of plant throughput continued throughout the year, with total carats produced increasing by 69% to 115 400 ct.

Dippenaar described the year-end results as “solid” and said that Petra remained on track to achieve its goal of producing five-million carats in 2019.

The company is investing in expansion projects at Finsch, Cullinan and Koffiefontein, which were all said to be progressing well. At Finsch, development and drilling contractor services had been put in place, with a total of 2 311 development meters achieved during the year.

At Cullinan, the C-Cut Phase 1 tunnel development yielded a total of 4 147 m for 2013, while raiseboring, which started during the year, delivered 626 m.

The revised mining layout of the planned new production areas to accelerate access to fresh kimberlite ore at Koffiefontein had been finalised, and detailed design had started.

Development work on the underground tunnel infrastructure was under way, while ore-handling conveyor installations servicing the 56 Level to 58 Level sub-level caves were in the final stages of commissioning. Group capital expenditure of $185.9-million for 2013, was largely driven by expansion and development activities.

Commenting on the diamond market, Dippenaar said that the overall price performance for 2014 was expected to remain steady as a result of constrained supply and a firmer US market, as well as continued growth in China, albeit at a lower rate than in recent years.

The diamond miner reported flat rough diamond prices in the first half of 2013, which firmed in the latter half of the year, resulting in stronger average rand value per carat prices realised across the group. The company will release its preliminary results for the year in September.