(IDEX Online) - Petra Diamonds says it expects production to fall this year, as it closes the loss-making Koffiefontein mine, in South Africa, and works towards re-opening the Williamson mine, in Tanzania, after the breach of a tailings dam.The UK-based miner is forecasting a total output of 2.75Mcts to 2.85Mcts for 2023, down from 3.35Mcts in 2022.But it forecasts an increase in 2024 to 3.00cts to 3.30Mcts and in 2025 to 3.60Mcts to 3.90Mcts.In addition to having two of its four mines out of action, Petra says Cullinan, in South Africa, it's highest-grossing mine, has recovered fewer exceptional stones (+$5m) so far during 2023 than in recent years.In 2022, five exceptional stones with a combined weight of 849.67 carats sold for $75,237,257, according to a company presentation published this week.In the first six months of 2023, two exceptionals, with a combined weight of 371.48 carats, have sold for 12,611,111.File pic shows the Cullinan Blue, named after the Petra mine where it was recovered in April 2021, which sold for almost $57.5m.