A rare rough ruby dubbed one of the world’s biggest has gone on display for the first time ahead of an auction in the Gulf emirate of Dubai. The 8,400-carat stone- nicknamed Burj Alhamal and weighing 2.8 kilogrammes (over six pounds)- was mined in Tanzania and made its debut at a Dubai hotel on Friday as part of SJ Gold and Diamond’s Callisto collection. The company says it is “one of the biggest” rough rubies in the world, with managing director Patrick Pilati calling it among the “rarest rubies ever found”. He said the Tanzanian stone was “not heated”, meaning “it has not been treated, so it…