BUZZ-Petra Diamonds: Worst day in 17 yrs on profit warning
** Petra Diamonds (Frankfurt: 908093 - news) -20.3 pct & on track for worst day in over 17 years
** Co sees full-year core earnings coming in about 10-15 percent below consensus, hurt by recent strengthening of the South African rand
** Petra, which flagged possible breach of two of its debt covenants in October, says it has started formal discussions with its lender group
** RBC Capital Markets cuts PT to 100p from 110p
** "The company still has sufficient liquidity ... but until we see a deleveraging on the balance sheet, financing concerns will continue to weigh," RBC says in a note
** Shares (Berlin: DI6.BE - news) of Petra lost half their value last year
** Stock bottom of LSE on Monday