Petra Diamonds’ rough prices decreased at its first tender of the fiscal year as the anticipated pickup in demand proved disappointing. The August trading session brought in $79.3 million from the sale of 696,194 carats, with like-for-like prices — those for similar categories of diamonds — falling 4.3% compared with May, the miner reported Friday. The slowdown was primarily due to flagging prices for rough between 2 and 10.8 carats, which dropped 14% on a like-for-like basis.