BUZZ-Petra Diamonds: Investec raises to "buy"
** Diamond miner Petra's shares up 5 pct, second-largest FTSE midcap gainer, after Investec (LSE: INVP.L - news) raises to "buy", cuts PT to 145p from 150p
** Investec analysts -- rated as four stars for recommendation accuracy on Petra, according to StarMine -- say current levels represent an attractive entry point, add rand weakness should help lower costs
** Shares (Berlin: DI6.BE - news) have fallen 44 pct since Aug 21 to Thursday's close, when the stock touched a 5-yr low
** Rand hit a record-low on Thursday (venkateshasoumithri.mamidipudi.thomsonreuters.com@reuters.net)