"Word has it old Monck had a way of forcing a djinn to serve him. There were rumours that he had more than one. He was said to keep them in bottles and make use of them when need arose. Three wishes from each genie, then it's free and escapes into its own dimension." - pg.251, The Last Wish (UK edition)
Geoffrey became famous because of his habit of trapping djinns in bottles and extracting three wishes out of each genie so ensnared. Apparently, he was able to conjure whirlwinds and hurricanes, as well as levitate with the help of his prisoners. He sealed each bottle with a sign depicting "a broken cross and a nine-pointed star".