Embedded in the wall of the Archbishop's Palace on Calle Hatunrumiyoc, a few blocks from the Plaza de Armas, the twelve-angled stone is one of the most photographed objects in Cusco. It's a single block of dark green andesite fitted precisely against twelve adjoining stones, each at a different angle, with joins so tight that no mortar was needed and a knife blade cannot be inserted between them.
The stone is part of the original Inca wall of the Roca palace, incorporated into the Spanish colonial building above it. You'll find it by following the crowd — a photographer usually marks the spot — or by counting the angles yourself once you're close enough to see the individual faces. The whole street has well-preserved Inca stonework worth examining, but the twelve-angled stone is the showpiece.