BUZZ-Petra: Warning on full-year results takes shine off

** Petra Diamonds Ltd (LSE: PDL.L - news) 's shares fall as much as 12 pct, after diamond miner warns that full-year results will be below market consensus due to variability in grade and production mix

** The stock, which was the second largest pct loser on the FTSE-250 Midcap Index on Thursday, has gained 11.6 pct since Petra first warned on results in Jan

** Petra, cuts its production guidance from 3.3 mln carats to 3.2 mln carats, just three months after it was increased, says it is sourcing material from mature diluted areas at its Finsch and Cullinan mines in South Africa

** Panmure Gordon analyst Alison Turner says will need to cut forecast of profit before tax and amortization of $124.2 million and sales of $470.3 million

** Although Petra's third-quarter production rises 6 pct to 791,443 carats, Turner says it is 4 pct short of his forecast

** Shares (Berlin: DI6.BE - news) in Petra, which has four producing mines in South Africa and one in Tanzania, touch a low of 162.27 pence ($2); analysts have a mean target price of $3.39 on the stock ($1 = 0.6745 pounds) (RM (LSE: RM.L - news) : esha.vaish.thomsonreuters.com@reuters.net)