The Twelve-Angled Stone of Cusco
The Twelve-Angled Stone (Piedra de los 12 Ángulos) is a single block of green diorite, approximately 3.84 meters high and weighing an estimated 1.5 to 2 tons, set into the lower course of a polygonal wall of the former Hatunrumiyoc palace on Calle Hatunrumiyoc in the historic center of Cusco. The stone has twelve interior angles, fitted to twelve neighboring stones, and thirteen exterior sides, and it is one of the most famous examples of imperial Inca stonework in existence. The wall in which the stone is set is a national cultural treasure of Peru and a central exhibit in any account of the architecture of imperial Cusco and of the imperial building style more generally.
Description of the stone
The stone is a roughly rectangular block of greenish-gray diorite, an intrusive igneous rock, with a slightly battered (inward-sloping) face that is characteristic of imperial Inca walls. The face of the block is divided into thirteen exterior sides, of which seven are vertical and six are at oblique angles, and the joint with each of the twelve surrounding stones is so close that a thin blade cannot be inserted into the gap. The block is one of the largest in the wall and one of the most complex in the imperial corpus, although the wall in which it sits contains numerous stones of comparable technical difficulty, including examples with nine, ten, and eleven sides.
The stone is set into the lower course of the polygonal wall, on a foundation course of larger, less carefully dressed stones. The lower course is the most visible, and the upper courses, including the more elaborate polygonal blocks that form the second story of the original palace, are largely concealed behind the later colonial plaster. The visible portion of the wall has been the subject of conservation work in the twentieth and twenty-first centuries, and the joints have been repointed with a fine sand-lime mortar in places where the original joints have been damaged by weathering or by the 1950 earthquake.
The wall and the Hatunrumiyoc palace
The wall of which the Twelve-Angled Stone forms part is the western wall of a small block that originally served as a residential and administrative compound of the Inca nobility. The block is associated in the chronicles with the Hatunrumiyoc, a high-ranking Inca, and is described in the colonial documents as a palace of a member of the imperial family. The block is rectangular, with interior courtyards, residential chambers, and an administrative zone, and it was originally built in the late fifteenth or early sixteenth century, during the reigns of the later Sapa Incas.
The polygonal wall is preserved in a length of approximately 36 meters along Calle Hatunrumiyoc, with the Twelve-Angled Stone as the central feature. The wall is interrupted by a trapezoidal doorway and by a small trapezoidal window, both of which are typical of the imperial building style. The upper stories of the building, of which the foundations are visible in the cellars of the colonial buildings, were largely demolished in the colonial period, and the surviving lower courses were incorporated into the foundations of the new colonial houses. The wall is now enclosed within the surviving colonial house (variously identified as the Casa Cabrera or the Casa del Almirante), whose ground floor preserves the imperial polygonal masonry and is open to visitors as part of the historic center of Cusco.
The significance of the stone
The Twelve-Angled Stone is a popular landmark and a symbol of Cusco and of imperial Inca stonework in general. It has appeared on stamps, on coins, on the cover of books, and in tourist literature, and it is the central feature of any tour of the imperial city. The stone is also cited in scholarly literature on Inca architecture as an example of the technical sophistication of the imperial building style, and it is frequently invoked in the debate over the technical and historical relationship between Inca construction and the construction of Machu Picchu and Ollantaytambo.
Some commentators have suggested that the Twelve-Angled Stone has astrological or astronomical significance, and that the angles of the block were deliberately set to mark a particular celestial event. The consensus among modern scholars is that the angles of the stone are a function of the polygonal style and of the technical requirements of fitting the block into the wall, and not of any astronomical symbolism. The block was shaped to fit the wall, and the wall was laid out so that the most convenient division of the polygonal jointing fell at the largest and most prominent stone.
The stone in relation to imperial style
The Twelve-Angled Stone is, with the curved polygonal wall of the Coricancha, one of the most celebrated examples of imperial Inca polygonal masonry. The two monuments are roughly contemporary, both dating to the late fifteenth or early sixteenth century, and they show the same technical features: the precise fitting of polygonal blocks, the inward batter of the wall face, the use of a foundation course of larger, less carefully dressed stones, and the incorporation of trapezoidal doorways and niches.
The block is also representative of the broader pattern of imperial stonework in which large, complex blocks are placed at the most visible points of a wall, with simpler polygonal blocks surrounding them. This pattern is found in many of the principal imperial buildings of Cusco, in the upper courses of the Sacsayhuamán walls, and in the principal buildings of Machu Picchu. The use of large and complex blocks at visible points is a deliberate display of technical skill and is one of the hallmarks of the imperial style.
Tool marks and the study of the stone
The Twelve-Angled Stone has been the subject of detailed technical study. Tool marks, including the small circular depressions of the sillares produced by stone hammers, are visible on the surface of the block and on many of its neighbors, and they have been the subject of photogrammetric surveys and microscopic analysis. The studies have confirmed that the stone was cut with stone hammers, with abrasives, and possibly with small copper or bronze tools, and that the final fit was achieved by patient abrasion with sand and water. The methods are consistent with the techniques used elsewhere in the imperial core and at the other major imperial sites.
The Italian architect Enrico Cancian and the North American researcher Jean-Pierre Protzen, both of whom have worked extensively on the polygonal masonry of the imperial period, have published technical descriptions of the stone, and the Cusco office of the Peruvian Ministry of Culture has conducted conservation work on the wall in the 1990s and 2000s. The stone is in good condition, although some of the joints have been damaged by the 1950 earthquake and by the gradual movement of the foundations of the colonial buildings that incorporate the wall.