(IDEX Online) - Petra reported an eight per cent slip in gross revenue for the six months to December 2020, down to $178m.But it said like-for-like prices at its tender earlier this month - 382,000 carats of rough sold for $30.5m - were up by eight per cent, and had returned to pre-COVID-19 levels. It raised $40m from the sale of the 85.6-ct Letlapa Tala Collection of blue diamonds (pictured) from its Cullinan mine in South Africa.The same mine also produced a 299-ct Type IIa white gem-quality diamond, which is expected to be sold next month.Production across all Petra's deposits for the period - Cullinan, Finsch, Koffiefontein and Williamson - was down 16 per cent to 1.74m carats, mainly because the Williamson mine in Tanzania remained on care and maintenance. Last year the UK-listed miner, which owed $650m, agreed a debt-for-equity restructuring deal with lenders, after failing to attract any buyers.Chief executive Richard Duffy noted that the Finsch mine, in South Africa, had suffered production problems because of waste ingress.However he said overall he was delighted that the proposed restructuring was due to be approved soon."This marks a significant milestone in putting the Company on a sustainable footing going forward," he said."The improvement in the market, with prices from our January sale now back at pre-COVID levels, is very encouraging and will provide some support as we look to optimise the value of our asset base."