Reading Ammianus Marcellinus has got me curious about the obelisks of Rome. This one is the Obelisk of Antinous, which the Emperor Hadrian commissioned in remembrance of his drowned sweetheart around 130 AD. It was carved in Egypt and delivered brand new to Rome, so it doesn't count among the ones taken from Egypt as loot. It's also known as the Pincian Obelisk after its location on the Pincian Hill. Read its history at the excellent Following Hadrian blog.