Right! The 12-sided stone is real!
Also called Hatunrumiyoc, the 12-sided stone is a real piece of stone cut to fit into a wall in a narrow pedestrian street in the city of Cuzco (or Cusco). The Inca were masters in stone cutting. The pieces in their stone walls are cut so precisely that you cannot fit a knife blade between them. There is no mortar between the stones.