BUZZ-Petra Diamonds: worst day in 2 yrs after Tanzania bars export
** Petra Diamonds (Frankfurt: 908093 - news) shares down 9 pct & set for worst day since Oct (Shenzhen: 000069.SZ - news) 2015 after falling as much as 28.3 pct
** Company says a parcel of diamonds from a mine it co-owns in Tanzania was blocked from export and that authorities were questioning some personnel
** Tanzania said on Saturday (Shenzhen: 002291.SZ - news) it planned to nationalise diamond consignment seized from the company
** Up to Friday's close, stock had fallen 44 pct this year
** Stock by far and away top pct loser on FTSE 250 & second most heavily traded on the index on Monday