Feb 25 (Reuters) - South African Miner Petra Diamonds Ltd reported 20 percent higher core earnings for the first half, helped by production at its recently purchased Finsch mine. Adjusted earnings before interest, taxes, depreciation and amortisation rose to $32.6 million for the six months to Dec. 31 from $27.1 million a year earlier. Petra, which has seven operating mines in South Africa and one in Tanzania, said revenue grew 63 percent to $151.7 million.